VOLTAGE DIVIDER PROVIDING SWITCHABLE RESOLUTION

Abstract: An analog signal is output from a voltage divider circuit. An adjustment in the resolution of that signal is provided by using a transistor switch to selectively connect and disconnect a resolution resistance in parallel with one of the resistances comprising the voltage divider circuit. The resulting resolution adjusted analog signal is then processed by an analog-to-digital converter. Alternatively, the analog signal output from the voltage divider circuit is applied to a scale changing amplification circuit implemented with an operational amplifier. An adjustment in the resolution of that signal is provided by using a transistor switch to selectively connect and disconnect a resolution resistance in parallel with a feedback resistance of the operational amplifier circuit. The resulting resolution adjusted analog signal is then processed by an analog-to-digital converter.

Description: VOLTAGE DIVIDER PROVIDING SWITCHABLE RESOLUTION BACKGROUND OF THE INVENTION Technical Field of the Invention The present invention relates to analog-to-digital (A/D) conversion and, in particular, to an apparatus for switching the resolution of an analog-to-digital converter input to provide for improved sensitivity. Description of Related Art Reference is now made to FIGURE 1 wherein there is shown a circuit diagram of a conventional analog-to- digital (A/D) converter based variable sensor 10 as is known in the prior art . A voltage supply 12 is included having a positive terminal 14 and a negative (ground) terminal 16. Connected between the terminals 14 and 16 is a voltage divider circuit 18 comprised of two series connected resistors 20. A first one of the resistors 20(1) is connected between the positive terminal 14 and a node 22. A second one of the resistors 20(2) is connected between the node 22 and the negative terminal 16. The node 22 is connected through a lead 24 to the input of an analog-to-digital converter 26. In one implementation, the variable being sensed by the sensor 10 is the voltage output from the voltage supply 12. The voltage supply 12 may accordingly comprise a battery. In another implementation, the variable being sensed by the sensor 10 is temperature. The second resistor 20(2) accordingly comprises a temperature sensitive thermistor. In either case, an analog signal having a voltage related (i.e., proportional) to the measured variable (voltage or temperature) is generated by the voltage divider circuit 18 at node 22 and applied via lead 24 to the input of an analog-to-digital converter 26. The voltage of the analog signal on lead 24 is then converted to a digital signal value for output from or for further processing by the analog-to-digital converter 26. In this regard it is recognized that the analog-to-digital converter 26 may comprise a discrete circuit element (as shown) or alternatively may comprise a part of a multi- function circuit element 28 (such as a micro-controller) . In many instances it is necessary to sense not only a general relative value of the variable (for example, in comparison to a given threshold) but also to sense the value of the variable with a higher degree of resolution for other purposes (for example, rate of change) . Take, for example, a battery voltage sensor where a user may need to know not only whether the battery level is high or low, but also how quickly the voltage is decreasing. To accomplish both goals, it .is often necessary for the sensed data concerning the variable to be evaluated with differing degrees of resolution. There is a need then for a circuit which supports an increase in the resolution of the analog signal output from commonly utilized voltage divider circuits prior to analog-to-digital conversion. SUMMARY OF THE INVENTION Adjustment in resolution of an analog signal output from a voltage divider circuit is provided in a first embodiment by selectively connecting and disconnecting a resolution resistance in parallel with one of the resistances comprising the voltage divider circuit. In a second embodiment, the analog signal output from the voltage divider circuit is applied to a scale changing amplification circuit, and the adjustment is provided by selectively connecting and disconnecting a resolution resistance in parallel with a feedback resistance of the amplification circuit to make an adjustment in gain (amplification) of the analog signal. In each embodiment, a transistor switch operating in response to an applied control signal effectuates the selective parallel connection and disconnection of the resolution resistance to provide switchable (high vs. low) resolution for the sensed variable. BRIEF DESCRIPTION OF THE DRAWINGS A more complete understanding of the method and apparatus of the present invention may be acquired by reference to the following Detailed Description when taken in conjunction with the accompanying Drawings wherein: FIGURE 1 (previously partially described) is a circuit diagram of a conventional prior art analog-to- digital (A/D) converter based variable sensor; FIGURE 2 is a circuit diagram for a first embodiment of an analog-to-digital (A/D) converter based variable sensor with resolution adjustment in accordance with the present invention; and FIGURE 3 is a circuit diagram for a second embodiment of an analog-to-digital (A/D) converter based variable sensor with resolution adjustment in accordance with the present invention. DETAILED DESCRIPTION OF THE DRAWINGS Reference is again made to FIGURE 1. The selection of the relative resistances for the individual resistors 20 within the voltage divider circuit 18 affects the resolution of the sensor 10 (i.e., its sensitivity to change in the sensed variable) . For an applied voltage V generated by the voltage supply 12, the voltage V for the analog signal generated by the voltage divider circuit 18 at node 22 is given by: N 7 = —- 1 — N (1) Rl + R, wherein: Rj_ is the resistance of the first resistor 20 (1) ; and R 2 is the resistance of the second resistor 20 (2) . Equation (1) illustrates that the larger the value of R 2 in comparison to the value of R τ , the more sensitive V is to changes in V and, hence, the greater the resulting resolution provided by V . This may be proven by reference to a specific example. Assume first that R a is equal to three ohms (3Ω) and that R 2 is equal to five ohms (5Ω) . Applying Equation (1) , for a V equal to ten volts (10V) , V is equal to 6.25V. If V were to now change to twelve volts (12v) , V would equal 7.5V. This is a change of 1.25V in V for a two volt swing in V. Now decrease the value of R x to two ohms (2Ω) . For a V still equal to ten volts (10V) , V is now equal to 7.14V. If V were to again change to twelve volts (12v) , V would equal 8.57V. This is a change of 1.43V in V for a two volt swing in V. Comparing the resulting changes in voltage shows that for the same two volt swing in V, a larger change in V (1.43V vs. 1.25V) occurs for a smaller value of R t (2Ω vs. 3Ω) relative to R 2 (5Ω) . Thus, the larger the value of R 2 relative to the value of R 1 results in increased resolution of the analog signal output from the voltage divider circuit 18 at node 22. This effect may be obtained by increasing the relative value of R 2 or decreasing the relative value of R x when a higher resolution is desired. Equation (1) further illustrates that, for a fixed V, the larger R 2 in comparison to R τ , the less sensitive V is to changes in R 2 and, hence, the greater the resulting resolution provided by V . This may be proven by reference to a specific example. Assume first that R x is equal to three ohms (3Ω) and that V is equal to ten volts (10V) . Applying Equation (1) , for R 2 equal to four ohms (4Ω) , V is equal to 5.71V. Now increase the value of R 2 to six ohms (6Ω) . V is now equal to 6.67V. This is a change of 0.96V in V for a two ohm swing in R 2 . If R- L were now decreased to one ohm (1Ω) , for R 2 again equal to four ohms (4Ω) , V is equal to 8.0V. Now again increase the value of R 2 to six ohms (6Ω) . V is now equal to 8.57V. This is a change of 0.57V in V for a two ohm swing in R 2 . Comparing the resulting changes in voltage shows that for the same two ohm swing in R 2 , a larger change in V (0.96V vs. 0.57V) occurs for a larger value of R l (3Ω vs. 1Ω) relative to R 2 . Thus, the smaller the value of R 2 relative to the value of R l results in increased resolution of the analog signal output from the voltage divider circuit 18 at node 22. This result may be obtained by increasing the relative value of Rj when a higher resolution is desired. Reference is now made to FIGURE 2 wherein there is shown a circuit diagram for a first embodiment of an analog-to-digital (A/D) converter based variable sensor 100 in accordance with the present invention. A voltage supply 112 is included having a positive terminal 114 and a negative (ground) terminal 116. Connected between the terminals 114 and 116 is a voltage divider circuit 118 comprised of a plurality (two shown) of series connected resistors 120. A first one of the resistors 120(1) is connected between the positive terminal 114 and a node 122. A second one of the resistors 120(2) is connected between the node 122 and the negative terminal 116. The node 122 is connected through a lead 124 to the input of an analog-to-digital converter 126 comprising a part of a multi-function circuit element 128 (such as a microcontroller) . The sensor 110 further includes a semiconductor transistor switch 130 comprising either a field effect transistor (FET) device (as illustrated) or a bi-polar transistor device (not shown) . The switch 130 includes a drain terminal 132 connected to the positive terminal 114, a gate terminal 134 connected to an output port 136 of the micro-controller 128, and a source terminal 138. The sensor 110 further includes a resolution resistor 140 connected between the source terminal 138 and the node 122 which is connected through the lead 124 to the input of the analog-to-digital converter 126. A signal selectively output by the micro-controller 128 from port 136 controls the operation of the switch 130 ("off" vs. "on") in effectively disconnecting and connecting, respectively, the resolution resistor 140 in parallel with the first resistor 120(1) between the positive terminal 114 and the node 122. When the switch 130 is turned on by the microcontroller 128 output signal, an effective first resistor 120(1) ' is formed by the resolution resistor 140 and the first resistor 120(1). The value of the effective first resistor 120(1) ' is given by: wherein: R λ is the resistance of the first resistor 120(1) ; R r is the resistance of the resolution resistor 140; and R eff is the resistance of the effective resistor 120 (1) ' . In one implementation, the variable being sensed by the sensor 110 is change in the voltage output from the voltage supply 112. In this case, the voltage supply 112 may accordingly comprise a battery. The sensor 110 operates in two resolution modes. In a low resolution mode (i.e., a mode where the sensor 110 is less sensitive to changes in the voltage output from the voltage supply 112) , the micro-controller 128 operates to output a signal from port 136 controlling the operation of the switch 130 ("off") in effectively disconnecting the resolution resistor 140 from the positive terminal 114. In a high resolution mode (i.e., a mode where the sensor 110 is more sensitive to changes in the voltage output from the voltage supply 112) , the micro-controller 128 operates to output a signal from port 136 controlling the operation of the switch 130 ("on") in effectively connecting the resolution resistor 140 to the positive terminal 114 and thus forming the effective resistor 120(1) '. In accordance with Equation (2) , the value of the effective resistor 120(1) ' is less than the value of the first resistor 120 (1) . As shown above in connection with the Equation (1) analysis for a given change in voltage V, with a larger value of the second resistor 120(2) (i.e., R 2 ) relative to the value of the first resistor 120(1) or its effective resistor 120(1) ' (i.e., R , an increase in resolution results for the analog signal output from the voltage divider circuit 118 at node 122 proving improved sensitivity. Such a larger relative value for the second resistor 120(2) occurs when the switch 130 is turned on and the resolution resistor 140 is connected. Although not specifically illustrated in FIGURE 1, it will be understood that an analogous operation of the sensor 110 may be obtained by using the switch 130 to selectively connect and disconnect the resolution resistor 140 in parallel with the second resistor 120(2) between the node 122 and the negative terminal 116. In this configuration, low resolution mode occurs when the switch 130 is turned "on" by the micro-controller 128, and high resolution mode occurs when the switch 130 is turned "off" by the micro-controller. In another implementation, the variable being sensed by the sensor 110 is temperature. The second resistor 120(2) in this implementation comprises a temperature sensitive thermistor. The sensor 110 again operates in two resolution modes. In a low resolution mode (i.e., a mode where the sensor 110 is less sensitive to changes in the resistance of the thermistor) , the micro-controller 128 operates to output a signal from port 136 controlling the operation of the switch 130 ("on") in effectively connecting the resolution resistor 140 to the positive terminal 114 and thus forming the effective resistor 120(1) ' . In a high resolution mode (i.e., a mode where the sensor 110 is more sensitive to changes in the voltage output from the voltage supply 112), the micro-controller 128 operates to output a signal from port 136 controlling the operation of the switch 130 ("off") in effectively disconnecting the resolution resistor 140 from the positive terminal 114. In accordance with Equation (2), the value of the effective resistor 120(1) ' is less that the value of the first resistor 120 (1) . As shown above in connection with the Equation (1) analysis for a given change in resistor R 2 , with a smaller value of the second resistor 120(2) (i.e., the thermistor R 2 ) relative to the value of the first resistor 120(1) or its effective resistor 120(1) ' (i.e., R 2 ) , an increase in resolution results for the analog signal output from the voltage divider circuit 118 at node 122 providing improved sensitivity. Such a smaller relative value for the second resistor 120(2) occurs when the switch 130 is turned off and the resolution resistor 140 is disconnected. Reference is now made to FIGURE 3 wherein there is shown a circuit diagram for a second embodiment of an analog-to-digital (A/D) converter based variable sensor 210 in accordance with the present invention. A voltage supply 212 is included having a positive terminal 214 and a negative (ground) terminal 216. Connected between the terminals 214 and 216 is a voltage divider circuit 218 comprised of a plurality (two shown) of series connected resistors 220. A first one of the resistors 220(1) is connected between the positive terminal 214 and a node 222. A second one of the resistors 220(2) is connected between the node 222 and the negative terminal 216. The node 222 is connected through a lead 224 to a first input 250 of an operational amplifier 252. A second input 254 of the operational amplifier 252 is connected to the negative (ground) terminal 216. An output 256 of the operational amplifier 252 is connected to the first input 250 through a feedback resistor 258. This effectively configures the operational amplifier to operate as a scale changer with respect to the received analog signal generated at the node 222. The output 256 of the operational amplifier 252 is further connected to the input of an analog-to-digital converter 226 comprising a part of a multi-function circuit element 228 (such as a micro-controller) . The sensor 210 further includes a semiconductor transistor switch 230 comprising either a field effect transistor (FET) device (as illustrated) or a bi-polar transistor device (not shown) . The switch 230 includes a drain terminal 232 connected to the output 256 of the operational amplifier 252, a gate terminal 234 connected to an output port 236 of the micro-controller 228, and a source terminal 238. The sensor 210 further includes a resolution resistor 140 connected between the source terminal 238 and the first input 250 of the operational amplifier 252 which is connected to node 222. A signal selectively output by the micro-controller 228 from port 236 controls the operation of the switch 230 ("off" vs. "on") in effectively disconnecting and connecting, respectively, the resolution resistor 140 in parallel with the feedback resistor 258 between the output 256 and the first input 250 of the operational amplifier 252. When the switch 230 is turned on by the micro-controller 228 output signal, an effective feedback resistor 258' is formed by the resolution resistor 140 and the feedback resistor 258. As the operational amplifier 252 is configured to operate as a scale changer, connecting and disconnecting the resolution resistor 140 in response to the micro-controller 228 output signal effectuates a gain change. The value of the effective feedback resistor 258' is given by : R f R r R* = Hζ <3) wherein: R f is the resistance of the feedback resistor 258; R r is the resistance of the resolution resistor 240; and R eff is the resistance of the effective feedback resistor 258 ' . In one implementation, the variable being sensed by the sensor 210 is change in the voltage output from the voltage supply 212. In this case, the voltage supply 212 may accordingly comprise a battery. The sensor 210 operates in two resolution modes. In a high resolution mode (i.e., a mode where the sensor 210 is more sensitive to changes in the voltage output from the voltage supply 212) , the micro-controller 228 operates to output a signal from port 236 controlling the operation of the switch 230 ("off") in effectively disconnecting the resolution resistor 240 from the output 256 of the operational amplifier 252. In accordance with Equation (3), the value of the feedback resistor 258 is more than the value of the effective feedback resistor 258'. Thus, a higher gain is applied by the scale changing operational amplifier 252 resulting in increased sensitivity to changes in the analog signal output from the voltage divider circuit 218 at node 222. In a low resolution mode (i.e., a mode where the sensor 210 is less sensitive to changes in the voltage output from the voltage supply 212), the micro-controller 228 operates to output a signal from port 236 controlling the operation of the switch 230 ("on") in effectively connecting the resolution resistor 240 in parallel with the feedback resistor 258 and thus forming the effective feedback resistor 258'. In accordance with Equation (3), the value of the effective feedback resistor 258 ' is less than the value of the feedback resistor 258. Thus, a lower (relative) gain is applied by the scale changing operational amplifier 252 resulting in decreased sensitivity to changes in the analog signal output from the voltage divider circuit 218 at node 222. Although preferred embodiments of the method and apparatus of the present invention have been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it will be understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the spirit of the invention as set forth and defined by the following claims .

Claims: WHAT IS CLAIMED IS: 1. A circuit, comprising: a voltage divider including a first resistor connected at a node in series with a second resistor; a resolution resistor; and a switch responsive to a control signal for selectively connecting and disconnecting the resolution resistor in parallel with the first resistor of the voltage divider. 2. The circuit as in claim 1 further including an analog-to-digital converter having an input connected to the node of the voltage divider. 3. The circuit as in claim 2 wherein the analog-to- digital converter is implemented within a microcontroller, and the micro-controller generates the control signal to change the resolution of an analog signal output from the node and processed by the analog-to-digital converter. 4. The circuit as in claim 1 wherein the second resistor comprises a thermistor. 5. The circuit as in claim 1 wherein: the switch comprises a semiconductor switch having a first terminal connected to receive the control signal, a second terminal connected to the resolution resistor, and a third terminal connected to an end of the first resistor opposite the connection to the node of the voltage divider; and an end of the resolution resistor opposite the connection with the second terminal is connected to the node of the voltage divider. 6. The circuit as in claim 5 wherein the semiconductor switch comprises a field effect transistor, and the first terminal comprises a gate, the second terminal comprises a source, and the third terminal comprises a drain. 7. A circuit, comprising: a voltage divider, comprising: a first resistor having a first end and a second end; and a second resistor having a first end and a second end, the first end of the second resistor connected to the second end of the first resistor at a node; a resolution resistor having a first end and a second end, the second end connected to the node; and a semiconductor switch having a first terminal connected to receive a control signal, a second terminal connected to the first end of the resolution resistor, and a third terminal connected to the first end of the first resistor, the control signal driving the semiconductor switch to selectively connect and disconnect the resolution resistor in parallel with the first resistor of the voltage divider. 8. The circuit as in claim 7 further including an analog-to-digital converter having an input connected to the node of the voltage divider. 9. The circuit as in claim 8 wherein the analog-to- digital converter is implemented within a microcontroller, and the micro-controller generates the control signal to change the resolution of an analog signal output from the node and processed by the analog-to-digital converter. 10. The circuit as in claim 7 wherein the second resistor comprises a thermistor. 11. The circuit as in claim 7 wherein the semiconductor switch comprises a field effect transistor, and the first terminal comprises a gate, the second terminal comprises a source, and the third terminal comprises a drain. 12. A circuit, comprising: a voltage divider including a first resistor connected at a node in series with a second resistor; an amplifier circuit having an input connected to the node and having a gain affecting feedback resistor; a resolution resistor; and a switch responsive to a control signal for selectively connecting and disconnecting the resolution resistor in parallel with the feedback resistor of the amplifier circuit. 13. The circuit as in claim 12 wherein the amplifier circuit comprises an operational amplifier circuit configured with the feedback resistor to operate as a scale changer. 14. The circuit as in claim 12 wherein: the switch comprises a semiconductor switch having a first terminal connected to receive the control signal, a second terminal connected to the resolution resistor, and a third terminal connected to an output of the amplifier circuit; and an end of the resolution resistor opposite the connection with the output of the amplifier circuit is connected to the input of the amplifier circuit. 15. The circuit as in claim 14 wherein the semiconductor switch comprises a field effect transistor, and the first terminal comprises a gate, the second terminal comprises a source, and the third terminal comprises a drain. 16. The circuit as in claim 12 further including an analog-to-digital converter having an input connected to an output of the amplifier circuit. 17. The circuit as in claim 16 wherein the analog- to-digital converter is implemented within a microcontroller, and the micro-controller generates the control signal to change the resolution of an analog signal output from the amplifier circuit and processed by the analog-to- digital converter. 18. The circuit as in claim 12 wherein the second resistor comprises a thermistor. 19. A circuit, comprising: a voltage divider, comprising: a first resistor having a first end and a second end; and a second resistor having a first end and a second end, the first end of the second resistor connected to the second end of the first resistor at a node; an operational amplifier having an input connected to the node, and having an output, the operational amplifier configured as a scale changer with a feedback resistor connected between its output and input; a resolution resistor having a first end and a second end, the second end connected to the input of the operational amplifier; and a semiconductor switch having a first terminal connected to receive a control signal, a second terminal connected to the first end of the resolution resistor, and a third terminal connected to the output of the operational amplifier, the control signal driving the semiconductor switch to selectively connect and disconnect the resolution resistor in parallel with the feedback resistor. 20. The circuit as in claim 19 wherein the semiconductor switch comprises a field effect transistor, and the first terminal comprises a gate, the second terminal comprises a source, and the third terminal comprises a drain. 21. The circuit as in claim 19 further including an analog-to-digital converter having an input connected to the output of the operational amplifier. 22. The circuit as in claim 21 wherein the analog- to-digital converter is implemented within a microcontroller, and the micro-controller generates the control signal to change the resolution of an analog signal output from the operational amplifier and processed by the analog-to-digital converter. 23. The circuit as in claim 19 wherein the second resistor comprises a thermistor.

Inventors: WAKEFIELD I NELSON, WAKEFIELD, I., NELSON
Applicants: ERICSSON GE MOBILE INC, ERICSSON INC.
Application year: 1998
Main classification: G01K7
IPC codes: G01K7, H03M1