# Patent

## Title
Circuits and methods for current measurements referred to a precision impedance

## Abstract

Circuitry and methods for obtaining accurate measurements of current supplied by an integrated circuit are provided. Current calculations are performed using information from a precision termination resistor and from the ratio relationship of two on-chip resistors. The invention provides a way to obtain accurate current measurements without the use of component trimming.

## Background
BACKGROUND OF THE INVENTION
This invention relates to driver circuitry. More particularly, this invention relates to driver circuitry that performs current measurements using a precision external resistor.
In the past, the semiconductor industry has utilized various configurations of “driver circuitry” for supplying power to loads that are external to an integrated circuit. Common examples of such external loads include transmission lines, communication systems, electric motors, and illumination systems. One characteristic of driver circuitry that is of interest to system designers is the amount of current the driver circuitry actually supplies to the external load. In certain applications, such as those involving power transmission or lighting systems, it is not necessary for the driver circuitry to obtain precise values of the supplied current. Other applications, however, such as communications systems, often rely on precise bias and modulation current measurements to properly function. Previously, precision current measurements have been made possible by “trimming” on-chip resistors and voltage references to obtain tolerance values suitable for current sensing. This method, however, is costly and time consuming.
In light of the foregoing, it would therefore be desirable to provide circuits and methods for accurately calculating the current supplied by driver circuitry without resorting to costly and time consuming trimming techniques.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide circuits and methods for precisely calculating the current supplied by driver circuitry without resorting to costly and time consuming resistor trimming techniques.
This and other objects are accomplished in accordance with the principles of the present invention by providing circuits and methods for precisely calculating the current supplied by driver circuitry without resorting to costly and time consuming resistor trimming techniques. Current calculations are performed using information from a precision termination resistor and from the ratio relationship of two on-chip resistors. The invention provides a way to obtain accurate current measurements without the use of component trimming.



BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects and advantages of the present invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which:
FIG. 1 is a schematic diagram of a driver circuit constructed in accordance with the principles of the present invention.
FIG. 2 is a schematic diagram of another driver circuit constructed in accordance with the principles of the present invention.



DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is an illustrative block diagram of a driver circuit 100 constructed in accordance with the principles of the present invention that can be used to obtain accurate measurements of supplied current. Driver circuit 100 includes a sourcing circuit 10, a sense resistor 15, an external termination resistor 25, analog to digital converters 40 and 55, a sinking circuit 45, and a modulation resistor 50.
Transmission line 30 and load 35 represent the external load driven by circuit 100. Capacitor 20 may also be included in circuit 100, if desired. Although other arrangements are possible, the components within dotted line 60 are typically constructed on an integrated circuit device.
As shown in FIG. 1, sourcing circuit 10 provides a substantially constant source current (Is) to sense resistor 15, termination resistor 25, and load 35 (through transmission line 30). Sourcing circuit 10 may be any circuit configuration suitable for providing a substantially constant current such as current mirror type bias circuitry.
The current supplied to load 35 may be varied by periodically switching sinking circuit 45 ON and OFF, allowing a modulation current (IM) to pass through it during an ON state, and acting as an open circuit during an OFF state. In some embodiments, sinking circuit 45 may be configured to turn ON and OFF partially to improve response time. Sinking circuit 45 may include any circuitry suitable for switching between ON and OFF states such as a transistor or armature type switch.
The value of modulation resistor 50 affects the operation of circuit 100 when sinking circuit 45 is switched ON. For example, when sinking circuit 45 turns ON and current flows across modulation resistor 50, the amount of current supplied to load 35 decreases in proportion to the resulting current divider network. Thus, if modulation resistor 50 has a value significantly less than load 35, the amount of current supplied to load 35 (IL) decreases significantly. On the other hand, if the value of modulation resistor 50 is much greater than load 35 (i.e., a factor of 10 or more), load current IL will only decrease somewhat. Thus, a circuit designer may select an approximate value for modulation resistor 50 with respect to load 35 to control the current differential applied across it. This allows circuit designers to define the range of current values supplied by circuit 100, such as those associated with the transmission of logic “high” or a logic “low,” signal to load 35.
Driver circuit 100 will operate without termination resistor 25. However, because transmission line 30 typically has a characteristic impedance similar to, but not perfectly matched with load 35, a portion of the transmitted signal is reflected back to the driver circuit, causing signal distortion. This problem may be corrected, however, by the addition of termination resistor 25 and capacitor 20, which allows current flow when the line voltage changes. These components help to absorb the reflected energy from load 35 so that high speed current cannot circulate back into the load. The present invention takes advantage of the fact that termination resistor 25 typically has significantly higher tolerance level than those found on an integrated circuit, usually around ±1%, which can be used to obtain accurate current measurements.
Measurement of source current IS and modulation current IM is performed using analog-to-digital converters (A/D converters) 40 and 55. This is accomplished by measuring the voltage drop across termination resistor 25, source resistor 15, and modulation resistor 50. The voltage drop across resistor 50 is divided by the voltage drop across resistor 15 and the voltage drop across termination resistor 25 divided by a reference voltage (not shown). These operations produce discrete values. In FIG. 1, these discrete values are depicted as ‘data’ outputs that are sent from A/D converters 40 and 55 to computational circuitry or software (not shown) that performs the mathematical operations necessary to determine the modulation current IM and source current IS (discussed in more detail below). Such circuitry may include, but is not limited to, a multiplication amplifier, a microprocessor, a programmable logic array, etc.
To calculate modulation current IM, the value of source current IS must be known. Generally speaking, source current IS is substantially equal to the current across termination resistor 25 (IT) when no current flows through capacitor 20. Current will only flow through capacitor 20 when the voltage across it changes. Because this occurs only for a very short duration at the instant sourcing circuit 45 switches from ON to OFF (and vice versa), an accurate measurement of source current IS may be obtained by measuring the voltage drop across precision termination resistor 25.
A measurement of the termination voltage (i.e, the voltage drop across termination resistor 25) may be used as a first step in determining the value of source current IS. For example, during operation, A/D converter 40 may take an analog voltage measurement of the termination voltage with respect to a reference voltage (not shown). This measurement, which is represented as XTR, is the termination voltage divided by the reference voltage. Accordingly, the variable XTR is a dimensionless ratio of these two voltages, having a value between 1 and 0 such that:

XTR=VT/VREF  (1)

Furthermore, it will be appreciated that the termination voltage is equal to the product of the termination current (IT) and the termination resistance (RT). Substituting this relationship into equation (1) gives the following:

XTR=(IT*RT)/VREF  (2)

Because source current IS is equivalent to termination current IT, equation (2) may be re-written as:

XTR=(IS*RT)/VREF  (3)

Solving equation (3) for source current IS gives the following:

IS=IT=XTR*VREF/RT.  (4)

Thus, as demonstrated above, circuit 100 may calculate the sourcing current IS as a function of precision termination resistor 25. In some embodiments of the present invention, equation 4 may be solved by an external computing device. The input to such a device is obtained from A/D converter 40.
It will be appreciated from the above that the accuracy of the sourcing current calculation is dependent only upon precision termination resistor 25 and an untrimmed internal voltage reference (not shown). Typically, these circuit components have tolerances of ±1% and ±5%, respectively. Thus, using the following well-known statistical error equation:

E=√{square root over (X2+Y2)}  (5)

where X represents the tolerance of termination resistor 25, and Y represents the tolerance of the voltage reference, it can be seen the accuracy of the source current measurement is approximately ±5%. This is a dramatic improvement over conventional systems that rely on measurements based on an untrimmed voltage reference and an untrimmed sense resistor 15. Because these components typically have tolerances of ±5% and ±20%, respectively, the overall accuracy of sourcing current measurements is approximately ±20%. Thus, the present invention provides sourcing current measurements that are at least four times more accurate than those provided by conventional systems.

The next step in calculating modulation current IM is to measure the voltage drop across modulation resistor 50 and compare it to the voltage drop across sensing resistor 15. A/D converter 55 accomplishes this and obtains a value for XMS, which is defined by the following:

XMS=VM/VS  (6)

where VM is the voltage across modulation resistor 50 and VS is the voltage across sensing resistor 15. According to Ohm's Law, each of these voltages are equal to the product of their respective current and resistance. Therefore:

VM=IM*RM;  (7)

and

VS=IS*RS  (8)

Substituting equations 7 and 8 into equation 6 yields:

XMS=(IM*RM)/(IS*RS)  (9)

Solving equation 9 for modulation current IM results in the following relationship:

IM=(XMS)*(RS/RM)*(IS)  (10)

To solve equation 10, a value of IS must be obtained. By substituting equation 4 into equation 10, the following expression is derived:

IM=(XMS*RS/RM)*(XTR*VREF/RT)  (11)

rearranging equation 11 to group like terms, the following is obtained:

IM=(XMS*XTR)(RS/RM)(VREF/RT)  (12)

Thus, it can be seen from equation 12, that the measurement of modulation current IM depends only on the reference voltage (VREF), termination resistor 25, and the ratio of modulation resistor 50 to sensing resistor 15. Because existing integrated circuit fabrication techniques permit the variance of the resistor ratio to be less than ±1%, the overall accuracy of the modulation current calculation remains approximately ±5%, without the use of trimming. This is a significant improvement over conventional measurement techniques which yield an overall accuracy of about ±20%.
Although driver circuit 100 may be used in a wide variety of applications that involve signal modulation, it is particularly useful for light-based modulation systems. In an optical communication system, for example, it is often necessary to quickly toggle a light source, such as a laser diode, to produce light pulses that are used as communication signals. To do this efficiently, circuit designers usually bias the light source such that it varies between two precise levels. One of the levels represents an optical “logic low,” during which a minimum optical signal is produced. The other level represents an optical “logic high” which produces an optical signal of sufficient strength to be differentiated from an optical “logic low” signal. The shorter this dynamic range is, the faster the light source can be varied between a “logic low” and a “logic high.” By having accurate measurements of bias and modulation currents, the driver circuitry can determine and optimize the extinction ratio (i.e., the amount of optical power need to produce a logic “high” signal compared to the optical power needed to produce a logic “low”) of the light source.
Accurately determining the operating currents and extinction ratio of circuit 100 has many advantages. For example, the average current required to maintain a constant average optical output power provides a good indication of the operational condition of the light source. For example, the turn ON threshold of a laser diode tends to degrade over time. By monitoring the average laser current, a communications system may accurately predict the imminent failure of the light source and provide a warning about impending failure so replacement can be made without loss of service.
Moreover, accurate measurements of the modulation and bias currents are useful for statistical process control during the final assembly of laser transceiver modules, where accurate measurement of laser setup conditions may give an early indication of manufacturing problems.
Another embodiment of the present invention may be used to calculate the modulation current IM that also yields an overall accuracy of ±5%. This approach is illustrated in FIG. 2 as driver circuit 200 which is similar to driver circuit 100 with the exception that circuit 200 employs a third A/D converter 60. A/D converter 60 measures the voltage drop across sensing resistor 15 with respect to a reference voltage (not shown) and generates a value represented as XSR. A/D converter 55 no longer uses the voltage across sensing resistor 15 as a reference. Next, A/D converter 40 operates as described above and determines XTR. A/D converter 55 then determines XMR, instead of XMS, where XMR is the voltage drop across modulation resistor 50 measured with respect to a reference voltage (not shown) so that:

XMR=VM/VREF  (13)

Using Ohm's law it can be shown that:

XMR=IM*RM/VREF  (14)

Solving for IM yields:

IM=XMR*VREF/RM  (15)

XSR may be obtained with A/D converter 60 and expressed using the following relationship:

XSR=IS*RS/VREF  (16)

Combining equations 3, 14, and 16 and rearranging gives the following:

IM=XMR*(XTR/XSR)*(RS/RM)*(VREF)/RT  (17)

Thus, it can be seen from equation 17, that the measurement of modulation current IM depends only on the reference voltage (VREF), termination resistor 25, and the ratio of modulation resistor 50 to sensing resistor 15. The readings obtained from A/D converters 40, 55, and 60 may then be sent to processing circuitry to perform the calculations as described above (not shown). Bias and modulation current information may be used to compute feedback information and the extinction ratio. These values may also be used internally by the driver circuit or externally for statistical analysis.
Persons skilled in the art will recognize that the present invention may be implemented using a variety of circuit configurations other than those shown and discussed above. For example, the present invention may employ some trimming on the internal reference voltage to improve its accuracy (e.g., to about ±1%). This further improves the accuracy of current measurements.
Moreover, termination resistor 25 need not be external to the integrated circuit, rather it may be fabricated on the integrated circuit and trimmed to the appropriate value. Furthermore, sinking circuit 45 may be replaced by or used in conjunction with a signal source that periodically impedes and allows current flow, thus providing modulation.
The present invention may also use data collecting components other than the A/D converters depicted in FIGS. 1 and 2 to implement different embodiments of the present invention. In addition, it will be appreciated that some or all of the resistors shown may be replaced by elements such as synthetic circuit components with impedances having a reactive component rather than elements with purely resistive attributes (e.g., a switched capacitor circuit). All such modifications will be recognized as within the scope of the present invention, which is limited only by the claims that follow.

## Claims

1. A circuit that determines a current supplied by an integrated circuit comprising:
a sensing impedance disposed on the integrated circuit;
a modulation impedance;
a first measurement device coupled to the modulation and sensing impedances and configured to measure a first voltage drop across each impedance;
a termination impedance;
a second measurement device coupled to the termination impedance and configured to measure a second voltage drop across the termination impedance; and
processing circuitry configured to receive the first and second voltage drops measured by the first and second measurement devices, respectively, and to calculate supplied current therefrom.

2. The circuit of claim 1 wherein the first measurement device is an analog to digital converter.

3. The circuit of claim 1 wherein the second measurement device is an analog to digital converter.

4. The circuit of claim 2 wherein the second analog to digital converter further comprises a trimmed voltage reference.

5. The circuit of claim 1 wherein the termination impedance is a precision resistor.

6. The circuit of claim 5 wherein the precision resistor is a resistor external to the integrated circuit.

7. The circuit of claim 5 wherein the precision resistor is a resistor internal to the integrated circuit.

8. The circuit of claim 1 further comprising a sinking circuit coupled to the modulation resistor.

9. The circuit of claim 1 wherein the supplied current is a modulation current.

10. A method for determining a current supplied by an integrated circuit comprising:
determining a voltage drop across a termination impedance with respect to a reference voltage;
comparing a voltage drop across a first impedance on the integrated circuit with a voltage drop across a second impedance on the integrated circuit, wherein the first impedance is different from the second impedance;
processing information obtained in the determining and comparing steps to obtain a value for the supplied current;
determining an impedance value of the first impedance, an impedance value of the second impedance, the voltage provided by the voltage reference, and the impedance value of the termination impedance; and
calculating a first value by dividing the value of the second impedance by the value of the first impedance, calculating a second value by dividing the value of the voltage drop across the termination impedance by the value of the reference voltage, calculating a third value by dividing the value of the reference voltage by the value of the termination impedance, calculating a fourth value by dividing the value of the voltage drop across the first impedance by the value of the reference voltage, calculating a fifth value by dividing the value of the voltage drop across the second impedance by the value of the reference voltage, and wherein the comparing further comprises calculating a sixth value by dividing the voltage drop across the first impedance by the voltage drop across the second impedance.

11. The method of claim 10 wherein the processing further comprises multiplying the first value, the second value, the third value, and the sixth value.

12. The method of claim 10 further comprising calculating a seventh value by dividing the fourth value by the fifth value.

13. The method of claim 11 further comprising wherein the processing further comprises multiplying the first value, the second value, the third value, and the seventh value.

14. Circuitry for determining a bias current provided by an integrated circuit comprising:
an off-chip impedance, not disposed on the integrated circuit, having a known resistance;
a source impedance disposed on the integrated circuit, wherein a first terminal of the source impedance is coupled to a first terminal of the off-chip impedance;
a modulation impedance disposed on the integrated circuit, wherein a second terminal of the modulation impedance is coupled to a second terminal of the off-chip impedance;
a first measurement device for measuring an off-chip voltage drop across the off-chip impedance with respect to a reference voltage;
a second measurement device for measuring a source voltage drop across the source impedance with respect to a modulation voltage drop across the modulation impedance; and
processing circuitry configured to determine the bias current based on the measurements of the first and second measurement devices.

15. A method for controlling the amount of output power provided by an integrated circuit, the method comprising:
measuring a first voltage drop across an off-chip impedance with respect to a reference voltage, wherein the off-chip impedance is not disposed on the integrated circuit, and the off-chip impedance has a known resistance;
measuring a second voltage drop across a source impedance with respect to a third voltage drop across a modulation impedance, wherein the source and modulation impedances are disposed on the integrated circuit;
determining a bias current based on the first, second and third voltage measurements; and
adjusting the bias current based on the determined bias current to control the amount of output power provided by the integrated circuit.