Imaging device, endoscope system, and imaging method

An imaging device includes a camera unit and a control unit. A first power source voltage is transferred from the control unit to the camera unit by a power source line and is input into the camera unit as a second power source voltage. The camera unit is configured to output a video signal, a reference signal having a reference voltage, and a voltage signal in accordance with the second power source voltage to a video signal line. The control unit is configured to measure a voltage value of each of the reference signal and the voltage signal. The control unit is configured to calculate a control value of the first power source voltage by using the measured voltage value.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to an imaging device, an endoscope system, and an imaging method.

DESCRIPTION OF RELATED ART

An endoscope system includes an endoscope (camera unit) and a main body, and the endoscope and the main body are connected to each other by a cable. An imager is mounted in the distal end of the endoscope. A power source voltage used for driving the imager is transferred from the main body to the distal end of the endoscope via the cable. Hereinafter, the power source voltage that has reached the distal end of the endoscope will be called a distal end voltage.

The power source voltage needs to be adjusted such that the distal end voltage has an appropriate value in order to drive the imager stably. However, the value of the distal end voltage may be less or greater than a voltage value recommended as an operation voltage value of the imager due to factors such as the length of the cable, deviations of the characteristics of the cable, and fluctuations of a current used for driving the imager. Hereinafter, the voltage value recommended as the operation voltage value of the imager will be called a recommended voltage value. As a result, problems may occur in driving the imager stably.

The prior art has improved the accuracy of circuit components or has performed an individual inspection of a power source at the time of production by considering a voltage drop caused by a cable and by considering fluctuations of current consumption caused by a change of a load of an imager in order to restrict the value of the distal end voltage within the recommended voltage value. Thus, the prior art has improved the accuracy of a power source voltage. However, in recent years, the diameter of a power source cable has needed to be reduced because of the demand for further reducing the diameter of an endoscope, and the distal end voltage is likely to be affected by the fluctuations of current consumption caused by the change of the load of the imager. Therefore, it has become difficult to restrict the value of the distal end voltage within the recommended voltage value.

A technique disclosed in Japanese Unexamined Patent Application, First Publication No. 2011-206333 provides a function of adjusting a power source voltage based on the distal end voltage. According to the technique, the distal end voltage is monitored at all times by using a dedicated cable for determining the distal end voltage, and the power source voltage is adjusted such that the distal end voltage has an appropriate value.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, an imaging device includes a camera unit and a control unit. The camera unit includes an image sensor, a reference voltage generation circuit, and a signal output circuit. The image sensor is configured to receive a first power source voltage transferred by a power source line as a second power source voltage and generate a video signal by using the second power source voltage. The reference voltage generation circuit is configured to generate a first reference voltage. The signal output circuit is configured to output the video signal, a reference signal having the first reference voltage, and a voltage signal having a first voltage indicating the second power source voltage to a video signal line. The control unit includes a signal reception circuit, a calculation circuit, a power source voltage generation circuit, and a voltage adjustment circuit. The signal reception circuit is configured to receive the video signal transferred by the video signal line, the reference signal having a second reference voltage, and the voltage signal having a second voltage and measure a value of the second reference voltage and a value of the second voltage. The received reference signal has the second reference voltage that has changed from the first reference voltage. The received voltage signal has the second voltage that has changed from the first voltage. The calculation circuit is configured to calculate a control value used for adjusting a value of the first power source voltage by using a value of the first reference voltage, the value of the second reference voltage, and the value of the second voltage. The power source voltage generation circuit is configured to generate the first power source voltage and output the generated first power source voltage to the power source line. The voltage adjustment circuit is configured to adjust the value of the first power source voltage by controlling the power source voltage generation circuit based on the control value.

According to a second aspect of the present invention, in the first aspect, the control unit may further include a current measurement circuit configured to measure a value of a current that flows through the power source line. The calculation circuit may be configured to calculate a resistance value of the power source line by using the value of the first power source voltage, the value of the first reference voltage, and the value of the current and calculate the control value by using the resistance value when the value of the second voltage is the same as the value of the second reference voltage.

According to a third aspect of the present invention, in the second aspect, a value of the second power source voltage is not necessarily within a range of a voltage of the video signal. The camera unit may further include a conversion circuit configured to convert the second power source voltage into the first voltage having a value within the range so as to generate the voltage signal. The calculation circuit may be configured to calculate the resistance value by using the value of the first power source voltage, the value of the first reference voltage, the value of the current, and a value indicating a ratio of the value of the second power source voltage to the value of the first voltage.

According to a fourth aspect of the present invention, in the first aspect, the calculation circuit may be configured to calculate a value of the second power source voltage corresponding to the first voltage by using the value of the first reference voltage, the value of the second reference voltage, and the value of the second voltage and calculate the control value by using the value of the second power source voltage.

According to a fifth aspect of the present invention, in the fourth aspect, a value of the second power source voltage is not necessarily within a range of a voltage of the video signal. The camera unit may further include a conversion circuit configured to convert the second power source voltage into the first voltage having a value within the range so as to generate the voltage signal. The calculation circuit may be configured to calculate the value of the second power source voltage by using the value of the first reference voltage, the value of the second reference voltage, the value of the second voltage, and a value indicating a ratio of the value of the second power source voltage to the value of the first voltage.

According to a sixth aspect of the present invention, in the fifth aspect, the control unit may further include a current measurement circuit configured to measure a value of a current that flows through the power source line. The calculation circuit may be configured to calculate a resistance value of the power source line by using the value of the first power source voltage, the value of the second power source voltage, and the value of the current and calculate the control value by using the resistance value.

According to a seventh aspect of the present invention, in the first aspect, the signal output circuit may be configured to output the video signal to the video signal line in a first period, output the reference signal to the video signal line in a second period different from the first period, and output the voltage signal to the video signal line in a third period different from any of the first period and the second period.

According to an eighth aspect of the present invention, an endoscope system includes a scope and the imaging device. The scope has a distal end and is to be inserted into a living body. The camera unit is disposed in the distal end.

According to a ninth aspect of the present invention, an imaging method is provided. The method includes receiving a first power source voltage transferred by a power source line as a second power source voltage; generating a video signal by using the second power source voltage; generating a first reference voltage; outputting the video signal, a reference signal having the first reference voltage, and a voltage signal having a first voltage indicating the second power source voltage to a video signal line; and receiving the video signal transferred by the video signal line, the reference signal having a second reference voltage, and the voltage signal having a second voltage. The received reference signal has the second reference voltage that has changed from the first reference voltage. The received voltage signal has the second voltage that has changed from the first voltage. The method includes measuring a value of the second reference voltage and a value of the second voltage; calculating a control value used for adjusting a value of the first power source voltage by using a value of the first reference voltage, the value of the second reference voltage, and the value of the second voltage; generating the first power source voltage; outputting the generated first power source voltage to the power source line; and adjusting the value of the first power source voltage by controlling a power source voltage generation circuit based on the control value.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described with reference to the drawings. Each of the embodiments will be described in detail by using an endoscope system as an example of an imaging device.

First Embodiment

FIG.1shows a configuration of an endoscope system1according to a first embodiment of the present invention. The endoscope system1shown inFIG.1includes an endoscope insertion unit2, a transmission cable3, an operation unit4, a connector unit5, a processor6, and a display device7. The endoscope insertion unit2, the transmission cable3, the operation unit4, and the connector unit5constitute a scope.

The endoscope insertion unit2includes an insertion unit2a. The insertion unit2ais part of the transmission cable3. The insertion unit2ais to be inserted inside a living body, which is a subject. The endoscope insertion unit2generates a video signal by imaging the inside of the subject. The endoscope insertion unit2outputs the generated video signal to the processor6. A camera unit9shown inFIG.2is disposed in a distal end2bof the insertion unit2a. In the insertion unit2a, the operation unit4is connected to the end part opposite the distal end2b. The operation unit4receives various operations for the endoscope insertion unit2from a user.

The transmission cable3connects the camera unit9and the connector unit5. The video signal generated by the camera unit9is output to the connector unit5via the transmission cable3.

The connector unit5is connected to the endoscope insertion unit2and the processor6. The connector unit5performs predetermined processing on the video signal output from the endoscope insertion unit2. The connector unit5outputs the video signal to the processor6.

The processor6performs image processing on the video signal output from the connector unit5. Furthermore, the processor6centrally controls the entire endoscope system1.

The display device7displays a video based on the video signal processed by the processor6. In addition, the display device7displays various kinds of information related to the endoscope system1.

The endoscope system1includes a light source device that generates illumination light emitted to the subject. The light source device is not shown inFIG.1.

FIG.2shows an internal configuration of the endoscope system1. The endoscope system1shown inFIG.2includes the camera unit9and the processor6. The camera unit9is disposed in the distal end2bof an endoscope. The operation unit4, the connector unit5, and the display device7are not shown inFIG.2.

The camera unit9includes a power source terminal90, a circuit unit91, a voltage generation circuit92, a level converter93, an imager94, a signal output circuit95, a buffer96, and a video terminal97. At least one of the circuit unit91, the voltage generation circuit92, the level converter93, the signal output circuit95, and the buffer96may be disposed in the imager94.

The processor6includes a voltage generation circuit60, a resistor61, an analog front end (AFE)62, a correction value calculation circuit63, and a voltage adjustment circuit64. The processor6is a control unit. All or part of the configuration of the processor6shown inFIG.2may be disposed in the operation unit4or the connector unit5.

The transmission cable3shown inFIG.1includes a power source line30and a video signal line31shown inFIG.2.

A schematic configuration of the endoscope system1will be described. The camera unit9and the processor6are connected to each other by both the power source line30that transfers a first power source voltage Vctrl and the video signal line31that transfers a video signal. The first power source voltage Vctrl transferred by the power source line30is input into the camera unit9as a second power source voltage Vcis. The imager94generates a video signal by using the second power source voltage Vcis. The voltage generation circuit92and the level converter93(reference voltage generation circuit) generate a first reference voltage Vref1. The signal output circuit95outputs the video signal, a reference signal having the first reference voltage Vref1, and a voltage signal having a first voltage Vfb_cis indicating the second power source voltage Vcis to the video signal line31.

The AFE62(signal reception circuit) receives the video signal, a reference signal, and a voltage signal transferred by the video signal line31. The reference signal has a second reference voltage Vref2that has changed from the first reference voltage Vref1by passing through the video signal line31. The voltage signal has a second voltage Vfb that has changed from the first voltage Vfb_cis by passing through the video signal line31. The AFE62measures a value of the second reference voltage Vref2and a value of the second voltage Vfb. The correction value calculation circuit63(calculation circuit) calculates a control value used for adjusting the value of the first power source voltage Vctrl by using the value of the first reference voltage Vref1, the value of the second reference voltage Vref2, and the value of the second voltage Vfb. The voltage generation circuit60(power source voltage generation circuit) generates the first power source voltage Vctrl and outputs the generated first power source voltage Vctrl to the power source line30. The voltage adjustment circuit64adjusts the value of the first power source voltage Vctrl based on the control value calculated by the correction value calculation circuit63.

A detailed configuration of the endoscope system1will be described. For example, the voltage generation circuit60is a voltage regulator. The voltage generation circuit60generates the first power source voltage Vctrl, which is a direct-current (DC) voltage.

The first power source voltage Vctrl generated by the voltage generation circuit60is output to the power source line30. The power source line30is a signal line disposed in the transmission cable3. The power source line30transfers the first power source voltage Vctrl output from the voltage generation circuit60to the camera unit9.

The power source terminal90is connected to the power source line30. The first power source voltage Vctrl transferred by the power source line30is input into the power source terminal90. The power source terminal90outputs the first power source voltage Vctrl to each circuit in the camera unit9as the second power source voltage Vcis. The second power source voltage Vcis is a power source voltage transferred by the power source line30to the camera unit9and is a voltage on a path including a path from the power source terminal90to the imager94. A voltage drop is generated due to a DC resistance of the power source line30, and the second power source voltage Vcis is attenuated. Therefore, the value of the second power source voltage Vcis is less than that of the first power source voltage Vctrl in the processor6.

The circuit unit91includes a circuit such as a timing generator or a phase-locked loop (PLL). The circuit unit91operates based on the second power source voltage Vcis.

The voltage generation circuit92and the level converter93constitute a reference voltage generation circuit. The voltage generation circuit92generates a first reference voltage based on the second power source voltage Vcis. For example, the voltage generation circuit92is constituted by a bandgap reference and can generate a voltage having a stable value.

The value of the second power source voltage Vcis is not within the range of the voltage of the video signal. The level converter93(conversion circuit) converts the second power source voltage Vcis into the first voltage Vfb_cis having a value within the range of the voltage of the video signal, thus generating the voltage signal.

For example, the value of the second power source voltage Vcis is about 3.3 V, and the range of the voltage of the video signal is about 100 mV. The range of the voltage of the video signal is a range from a minimum allowable voltage of the video signal to a maximum allowable voltage of the video signal. In order to fit the value of the second power source voltage Vcis into this range, the level converter93converts the second power source voltage Vcis into the first voltage Vfb_cis. The value of the first voltage Vfb_cis is less than that of the second power source voltage Vcis.

The second power source voltage Vcis and the first voltage Vfb_cis meet a condition shown in the following Expression (1). A coefficient k in Expression (1) indicates a ratio of the value of the second power source voltage Vcis to the value of the first voltage Vfb_cis. The coefficient k is a predetermined value greater than 1.
Vfb_cis=Vcis×1/k(1)

Similarly to the above, the level converter93converts the first reference voltage generated by the voltage generation circuit92into the first reference voltage Vref1having a value within the range of the voltage of the video signal, thus generating the reference signal. The level converter93outputs both the reference signal having the first reference voltage Vref1and the voltage signal having the first voltage Vfb_cis to the signal output circuit95.

The imager94is an image sensor such as a complementary metal-oxide semiconductor (CMOS) sensor. The imager94includes a plurality of pixels and generates a video signal having a voltage generated based on the second power source voltage Vcis. The imager94outputs the video signal to the signal output circuit95.

The signal output circuit95outputs an analog signal output from the imager94or the level converter93to the video signal line31via the buffer96and the video terminal97. The signal output circuit95outputs the video signal output from the imager94to the video signal line31in a first period. The signal output circuit95outputs the reference signal output from the level converter93to the video signal line31in a second period different from the first period. The signal output circuit95outputs the voltage signal output from the level converter93to the video signal line31in a third period different from any of the first period and the second period.

For example, the first period is a period during which the imager94outputs the video signal. An entire period of the second period and the third period is all or part of a period excluding the first period. For example, the second period and the third period are included in a blanking period during which the imager94stops the output of the video signal. The second period and the third period are one or both of a horizontal blanking period and a vertical blanking period.

FIG.3shows a waveform of each signal output from the signal output circuit95. Waveforms of the video signal, the reference signal, and the voltage signal are shown inFIG.3. The horizontal direction inFIG.3indicates time, and the vertical direction inFIG.3indicates a voltage value.

The signal output circuit95outputs the reference signal having the first reference voltage Vref1after outputting the video signal. The signal output circuit95outputs the voltage signal having the first voltage Vfb_cis after outputting the reference signal.

The order of the reference signal and the voltage signal is not limited to that shown inFIG.3. The signal output circuit95may output the reference signal after outputting the voltage signal.

The analog signal output from the signal output circuit95is input into the video terminal97via the buffer96. The video terminal97is connected to the video signal line31. The video terminal97sequentially outputs the video signal, the reference signal, and the voltage signal to the video signal line31. The video signal line31is a signal line disposed in the transmission cable3. The video signal line31transfers the video signal to the processor6in the first period, transfers the reference signal to the processor6in the second period, and transfers the voltage signal to the processor6in the third period.

The resistor61is connected to the video signal line31. The resistor61is a terminal resistor. The AFE62is connected to the video signal line31. The video signal is input into the AFE62in the first period, the reference signal is input into the AFE62in the second period, and the voltage signal is input into the AFE62in the third period.

FIG.4shows a waveform of each signal received by the AFE62, which is a signal reception circuit. Waveforms of the video signal, the reference signal, and the voltage signal are shown inFIG.4. The horizontal direction inFIG.4indicates time, and the vertical direction inFIG.4indicates a voltage value.

The AFE62receives the reference signal having the second reference voltage Vref2after receiving the video signal. The AFE62receives the voltage signal having the second voltage Vfb after receiving the reference signal.

A voltage drop is generated due to the DC resistance of the video signal line31, and the reference signal and the voltage signal are attenuated. Therefore, the first reference voltage Vref1in the camera unit9changes to the second reference voltage Vref2, and the first voltage Vfb_cis in the camera unit9changes to the second voltage Vfb. The value of the second reference voltage Vref2is less than that of the first reference voltage Vref1. In addition, the value of the second voltage Vfb is less than that of the first voltage Vfb_cis.

It is assumed that the attenuation rate of the reference signal is the same as that of the voltage signal. Therefore, a ratio (Vref1/Vref2) of the value of the first reference voltage Vref1to the value of the second reference voltage Vref2is the same as that (Vfb_cis/Vfb) of the value of the first voltage Vfb_cis to the value of the second voltage Vfb.

The AFE62includes an analog-to-digital converter and converts the received analog signal into a digital signal. The AFE62processes the digital signal of each of the video signal, the reference signal, and the voltage signal. For example, the AFE62performs predetermined signal processing on the video signal. In addition, the AFE62measures the value of the second reference voltage Vref2of the received reference signal and measures the value of the second voltage Vfb of the received voltage signal. The AFE62outputs the value of the second reference voltage Vref2and the value of the second voltage Vfb to the correction value calculation circuit63.

The correction value calculation circuit63calculates the value of the second power source voltage Vcis corresponding to the first voltage Vfb_cis by using the value of the first reference voltage Vref1, the value of the second reference voltage Vref2, and the value of the second voltage Vfb. Specifically, the correction value calculation circuit63calculates the value of the second power source voltage Vcis by using the value of the first reference voltage Vref1, the value of the second reference voltage Vref2, the value of the second voltage Vfb, and the value of a coefficient k. For example, the correction value calculation circuit63calculates the value of the second power source voltage Vcis in accordance with the following Expression (2).
Vcis=Vfb_cis×k=(Vref1/Vref2)×Vfb×k(2)

The correction value calculation circuit63calculates a control value (correction value) of the first power source voltage Vctrl by using the value of the second power source voltage Vcis. The correction value calculation circuit63outputs the calculated control value to the voltage adjustment circuit64. The voltage adjustment circuit64controls the voltage generation circuit60based on the control value, thus adjusting the value of the first power source voltage Vctrl to be generated by the voltage generation circuit60.

The voltage adjustment circuit64adjusts the value of the first power source voltage Vctrl such that the value of the second power source voltage Vcis input into the power source terminal90is a recommended voltage value for the operation of the imager94. For example, the recommended voltage value is 3.3 V. In order for the value of the second power source voltage Vcis to be 3.3 V, a condition shown in the following Expression (3) needs to be met.
Vctrl(tn)=Vctrl(tn−1)+(3.3−Vcis(tn−1))  (3)

A value Vctrl(tn) in Expression (3) indicates the value of the first power source voltage Vctrl at a time point tn. A value Vctrl(tn−1) in Expression (3) indicates the value of the first power source voltage Vctrl at a time point tn−1 before the time point tn. In addition, the value Vctrl(tn−1) indicates the value of the first power source voltage Vctrl adjusted by the voltage adjustment circuit64last time. An initial value Vctrl(t0) of the first power source voltage Vctrl at a time point t0is a predetermined value. A value Vcis(tn−1) in Expression (3) indicates the value of the second power source voltage Vcis at the time point tn−1. The correction value calculation circuit63calculates the value Vctrl(tn) of the first power source voltage Vctrl in accordance with Expression (3) and outputs the value Vctrl(tn) to the voltage adjustment circuit64.

When the amount of the voltage drop in the power source line30becomes large, the value of the second power source voltage Vcis becomes small. Therefore, the voltage adjustment circuit64increases the value of the first power source voltage Vctrl. When the amount of the voltage drop in the power source line30becomes small, the value of the second power source voltage Vcis becomes large. Therefore, the voltage adjustment circuit64decreases the value of the first power source voltage Vctrl.

In the first embodiment, the endoscope system1can monitor the power source voltage (second power source voltage Vcis) provided to the imager94. The video signal line31transfers the reference signal and the voltage signal used for adjusting the value of the first power source voltage Vctrl. Therefore, a dedicated cable used for transferring the reference signal and the voltage signal is unnecessary, and miniaturization of the camera unit9is not prevented.

The endoscope system1calculates the value of the second power source voltage Vcis based on a relationship of the amount between the first reference voltage Vref1having a known value and the second reference voltage Vref2that has been attenuated in the video signal line31. Therefore, the endoscope system1can calculate the value of the second power source voltage Vcis based on an analog signal with high accuracy.

Since the AFE62that processes the video signal measures the value of the second reference voltage Vref2and the value of the second voltage Vfb, an increase of the circuit scale is restricted. The endoscope system1can adjust the value of the first power source voltage Vctrl by following a change of the voltage drop in the power source line30.

Second Embodiment

FIG.5shows an internal configuration of an endoscope system1aaccording to a second embodiment of the present invention. The same configuration as that shown inFIG.2will not be described. The endoscope system1ashown inFIG.5includes a camera unit9and a processor6a.

The camera unit9is the same as that shown inFIG.2. The processor6aincludes a voltage generation circuit60, a resistor61, an AFE62, a correction value calculation circuit63a, a voltage adjustment circuit64, and a current measurement circuit65. All or part of the configuration of the processor6ashown inFIG.5may be disposed in the operation unit4or the connector unit5.

The current measurement circuit65measures a value of a current (DC current) that flows through the power source line30. The current measurement circuit65outputs the measured value to the correction value calculation circuit63a.

The correction value calculation circuit63acalculates a resistance value Rvdd(tn−1) of the power source line30at a time point tn−1 by using a value Vctrl(tn−1) of the first power source voltage Vctrl at the time point tn−1, a value Vcis(tn−1) of the second power source voltage Vcis at the time point tn−1, and a value Ivdd(tn−1) of the current at the time point tn−1. For example, the correction value calculation circuit63acan calculate the resistance value Rvdd(tn−1) in accordance with the following Expression (4).
Rvdd(tn−1)=(Vctrl(tn−1)−Vcis(tn−1)/Ivdd(tn−1)  (4)

The value Vctrl(tn) of the first power source voltage Vctrl at a time point tn, the value Vcis(tn−1) of the second power source voltage Vcis at the time point tn−1, the resistance value Rvdd(tn−1) of the power source line30at the time point tn−1, and the value Ivdd(tn−1) of the current at the time point tn−1 meet a condition shown in the following Expression (5).
Vcis(tn−1)=Vctrl(tn)−(Rvdd(tn−1)×Ivdd(tn−1))  (5)

Accordingly, in order for the value of the second power source voltage Vcis to be 3.3 V, a condition shown in the following Expression (6) needs to be met.
Vctrl(tn)=3.3+(Rvdd(tn−1)×Ivdd(tn−1))  (6)

The correction value calculation circuit63acalculates the value Vctrl(tn) of the first power source voltage Vctrl in accordance with Expression (6) and outputs the value Vctrl(tn) to the voltage adjustment circuit64. The voltage adjustment circuit64controls the voltage generation circuit60based on the value Vctrl(tn).

In the second embodiment, the endoscope system1acan monitor the power source voltage provided to the imager94and does not prevent miniaturization of the camera unit9similarly to the endoscope system1according to the first embodiment. In addition, the endoscope system1acan calculate the resistance value of the power source line30and can calculate the amount of the voltage drop in the power source line30. The endoscope system1acontrols the voltage generation circuit60based on the amount and therefore can directly adjust the value of the first power source voltage to be generated by the voltage generation circuit60.

Third Embodiment

FIG.6shows an internal configuration of an endoscope system1baccording to a third embodiment of the present invention. The same configuration as that shown inFIG.5will not be described. The endoscope system1bshown inFIG.6includes a camera unit9band a processor6b.

The camera unit9bincludes a power source terminal90, a circuit unit91, a voltage generation circuit92, a level converter93b, an imager94, a signal output circuit95, a buffer96, and a video terminal97. At least one of the circuit unit91, the voltage generation circuit92, the level converter93b, the signal output circuit95, and the buffer96may be disposed in the imager94.

The processor6bincludes a voltage generation circuit60, a resistor61, an AFE62, a correction value calculation circuit63b, a voltage adjustment circuit64, and a current measurement circuit65. All or part of the configuration of the processor6bshown inFIG.6may be disposed in the operation unit4or the connector unit5.

The level converter93bconverts the second power source voltage Vcis into a first voltage Vfb_cis. Specifically, the level converter93bconverts the second power source voltage Vcis into a first voltage Vcis/m and a first voltage Vcis/n. Each of a coefficient m and a coefficient n is a predetermined value greater than 1. Each of the coefficient m and the coefficient n indicates a ratio of the value of the second power source voltage Vcis to the value of the first voltage. The value of the first voltage Vcis/m and the value of the first voltage Vcis/n are less than the value of the second power source voltage Vcis. For example, the value of the coefficient n is greater than that of the coefficient m, and the value of the first voltage Vcis/n is less than that of the first voltage Vcis/m. The first voltage Vfb_cis indicates the difference between the first voltage Vcis/m and the first voltage Vcis/n.

On the other hand, the level converter93bconverts the first reference voltage generated by the voltage generation circuit92into a first reference voltage Vref1, thus generating a reference signal. The first reference voltage Vref1indicates the difference between a predetermined voltage Vbgr and the first voltage Vcis/n. The level converter93boutputs the reference signal having the first reference voltage Vref1and a voltage signal having the first voltage Vfb_cis to the signal output circuit95.

The signal output circuit95outputs the video signal output from the imager94to the video signal line31in a first period. The signal output circuit95outputs the reference signal output from the level converter93bto the video signal line31in a second period different from the first period. The signal output circuit95outputs the voltage signal output from the level converter93bto the video signal line31in a third period different from any of the first period and the second period.

The AFE62receives the video signal, the reference signal, and the voltage signal and measures a value of the second reference voltage Vref2and a value of the second voltage Vfb. The AFE62outputs the value of the second reference voltage Vref2and the value of the second voltage Vfb to the correction value calculation circuit63b. The current measurement circuit65measures a value of a current that flows through the power source line30and outputs the measured value to the correction value calculation circuit63b.

The correction value calculation circuit63bdetermines whether the value of the second voltage Vfb is the same as that of the second reference voltage Vref2. When the value of the second voltage Vfb is the same as that of the second reference voltage Vref2, the correction value calculation circuit63bcalculates the resistance value of the power source line30by using the value of the first power source voltage Vctrl, the value of the first reference voltage Vref1, and the value of the current. The correction value calculation circuit63bcalculates a control value of the first power source voltage Vctrl by using the calculated resistance value. The correction value calculation circuit63boutputs the calculated control value to the voltage adjustment circuit64. The voltage adjustment circuit64controls the voltage generation circuit60based on the control value, thus adjusting the value of the first power source voltage Vctrl to be generated by the voltage generation circuit60.

When the value of the second voltage Vfb is the same as that of the second reference voltage Vref2, the second power source voltage Vcis has a value that is based on the value of the first reference voltage Vref1and the coefficient m. A method of acquiring the value of the second power source voltage Vcis will be described by usingFIGS.7to12.

FIGS.7,9, and11show a waveform of each signal output from the signal output circuit95.FIGS.8,10, and12show a waveform of each signal received by the AFE62, which is a signal reception circuit. Waveforms of the video signal, the reference signal, and the voltage signal are shown in each drawing. The horizontal direction in each drawing indicates time, and the vertical direction in each drawing indicates a voltage value.

A voltage drop is generated due to the DC resistance of the video signal line31, and the reference signal and the voltage signal are attenuated. Therefore, the first reference voltage Vref1in the camera unit9bchanges to the second reference voltage Vref2. The value of the second reference voltage Vref2is less than that of the first reference voltage Vref1. In addition, the first voltage Vcis/m in the camera unit9bchanges to a second voltage Vfb_m, and the first voltage Vcis/n in the camera unit9bchanges to a second voltage Vfb_n. The second voltage Vfb indicates the difference between the second voltage Vfb_m and the second voltage Vfb_n. The value of the second voltage Vfb is less than that of the first voltage Vfb_cis.

FIG.7shows an example in which the value of the first voltage Vfb_cis is less than that of the first reference voltage Vref1. In this case, the value of the second voltage Vfb is less than that of the second reference voltage Vref2as shown inFIG.8.

FIG.9shows an example in which the value of the first voltage Vfb_cis is greater than that of the first reference voltage Vref1. In this case, the value of the second voltage Vfb is greater than that of the second reference voltage Vref2as shown inFIG.10.

In the examples shown inFIG.8andFIG.10, the value of the second voltage Vfb is different from that of the second reference voltage Vref2. The correction value calculation circuit63boutputs the control value of the first power source voltage Vctrl causing the value of the second voltage Vfb to match the value of the second reference voltage Vref2to the voltage adjustment circuit64.

For example, when the value of the second voltage Vfb is less than that of the second reference voltage Vref2, the correction value calculation circuit63boutputs the control value to increase the value of the first power source voltage Vctrl to the voltage adjustment circuit64. When the value of the second voltage Vfb is greater than that of the second reference voltage Vref2, the correction value calculation circuit63boutputs the control value to decrease the value of the first power source voltage Vctrl to the voltage adjustment circuit64.

The voltage adjustment circuit64controls the voltage generation circuit60based on the control value, thus adjusting the value of the first power source voltage Vctrl to be generated by the voltage generation circuit60. The above-described control is repeated until the value of the second voltage Vfb matches the value of the second reference voltage Vref2.

FIG.11shows an example in which the value of the first voltage Vfb_cis is the same as that of the first reference voltage Vref1. In this case, the value of the second voltage Vfb is the same as that of the second reference voltage Vref2as shown inFIG.12.

When the value of the second voltage Vfb is the same as that of the second reference voltage Vref2, the value of the first voltage Vfb_cis is the same as that of the first reference voltage Vref1and the value of the first voltage Vcis/m is the same as that of the predetermined voltage Vbgr. Therefore, the value of the second power source voltage Vcis meets a condition shown in the following Expression (7). The correction value calculation circuit63bcan acquire the value of the second power source voltage Vcis shown in Expression (7).
Vcis=Vbgr×m(7)

When the value of the second voltage Vfb at a time point tn−1 is the same as that of the second reference voltage Vref2at the time point tn−1, the correction value calculation circuit63bcalculates a resistance value Rvdd(tn−1) of the power source line30at the time point tn−1 by using a value Vctrl(tn−1) of the first power source voltage Vctrl at the time point tn−1, a value Vcis(tn−1) of the second power source voltage Vcis at the time point tn−1, and a value Ivdd(tn−1) of the current at the time point tn−1. For example, the correction value calculation circuit63bcan calculate the resistance value Rvdd(tn−1) in accordance with Expression (4) described above.

The correction value calculation circuit63bcalculates a value Vctrl(tn) of the first power source voltage Vctrl in accordance with Expression (6) described above by using the resistance value Rvdd(tn−1) and the value Ivdd(tn−1) at the time point tn−1. The correction value calculation circuit63boutputs the calculated value Vctrl(tn) to the voltage adjustment circuit64. The voltage adjustment circuit64controls the voltage generation circuit60based on the value Vctrl(tn).

In the third embodiment, the endoscope system1bcan monitor the power source voltage provided to the imager94and does not prevent miniaturization of the camera unit9bsimilarly to the endoscope system1according to the first embodiment. When the value of the second voltage Vfb matches the value of the second reference voltage Vref2, the endoscope system1bcan calculate the resistance value of the power source line30and can calculate the amount of the voltage drop in the power source line30. The endoscope system1bcontrols the voltage generation circuit60based on the amount and therefore can directly adjust the value of the first power source voltage to be generated by the voltage generation circuit60.