# Patent

## Title
Method and apparatus for measuring the common-mode component of a differential signal

## Abstract

A common-mode detector includes a first difference amplifier that is connected to compare a first input voltage with a feedback voltage to provide a first result, a second difference amplifier that is connected to compare a second input voltage with the feedback voltage to provide a second result, and a feedback amplifier that is connected to drive the feedback voltage to a level that is substantially the average of the first and second input voltages in response to receiving the first and second results.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to electronic systems and, more particularly, to common-mode detectors.
2. Description of the Related Art
Automatic test equipment (ATE) is used to test electric circuits prior to completing their manufacture. Although differential component detectors are relatively common in ATE, the use of common-mode detectors is relatively new. One such common-mode detector is described by C. Bishop in U.S. Pat. No. 6,281,699. A window comparator generates an output indicative of whether the common-mode signal is above, within, or below predetermined thresholds to determine a common-mode component. Two buffers and a resistor pair are provided with the window comparator to extract the common-mode signal and reduce loading effects on input signals from a device under test (DUT).
Unfortunately, a substantial current can flow through the resistor pair when the comparator's input signals are separated by a large differential voltage. Thus, the buffers should be designed to source and sink the maximum expected current that can flow. Additionally, the resistors must be made low in value in order to operate the circuit with high bandwidth. This adds to the problem of excessive current flow and can also cause significant nonlinearity in the response of the buffers.
There still exists a need, therefore, for a common-mode detector that does not suffer from large currents resulting from large input differential voltages and which does not suffer from high output impedance that precludes high bandwidth operation.
SUMMARY OF THE INVENTION
A common-mode detector is disclosed for determining the common-mode component of a differential signal. It includes a first difference amplifier that is connected to compare a first input voltage, preferably a noninverted component of the differential signal, with a feedback voltage to provide a first result. A second difference amplifier is connected to compare a second input voltage, preferably an inverted component of the differential signal, with the feedback voltage to provide a second result. A feedback amplifier is connected to drive the feedback voltage to a level that is substantially the average of the first and second input voltages in response to receiving the first and second results.
In one embodiment, a method is described for detecting the common-mode component. A voltage difference between the noninverted component of the differential signal and a feedback signal is converted to a first differential current signal. A voltage difference between the inverted component of the differential signal and the feedback signal is converted to a second differential current signal. The first and second differential current signals are compared to generate the feedback signal, which settles to a voltage indicative of the common-mode component of the differential signal.



BRIEF DESCRIPTION OF THE DRAWINGS
The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principals of the invention. Moreover, in the figures, like reference numerals designate corresponding parts throughout the different views.
FIG. 1 is a block diagram of one embodiment of the invention with two differential amplifiers coupled to differential inputs VA and VB, respectively, with their outputs coupled to an op-amp for feedback.
FIG. 2 is a schematic illustrating one implementation of the circuit illustrated in FIG. 1, which has two transistor pairs for implementing the differential amplifiers.
FIG. 3 is a flow diagram of an embodiment that compares input signals VA and VB to an output voltage VO.



DETAILED DESCRIPTION OF THE INVENTION
A common-mode detector, in one embodiment, has two difference amplifiers for comparing each component of a differential signal to a feedback voltage. Each difference amplifier outputs a differential result that varies with the voltage difference between its respective differential signal component and the feedback signal. A feedback amplifier compares first and second results to generate the feedback voltage that is indicative of the difference between the first and second voltages. The feedback voltage is driven by the feedback amplifier to a voltage representing the common-mode component of the differential signal.
FIG. 1 illustrates a common-mode detector 100 that has, in one embodiment, two differential transconductance amplifiers and a feedback amplifier, preferably an operational amplifier (OA1), to generate a voltage proportional to the common-mode component of a differential signal. The noninverting and inverting components of the differential signal are received at input terminals VA and VB, respectively, of the detector 100. A first input VP1 of a differential transconductance amplifier gm1 is coupled to terminal VA to receive the noninverting component of the differential signal. A second input VN1 of gm1 is coupled to the detector's output at terminal VO for feedback. A second differential transconductance amplifier gm2 has first and second inputs VP2 and VN2, with VP2 coupled to terminal VB to receive the inverting component of the differential signal, and VN2 coupled to terminal VO to receive feedback from the output of the detector 100. Consequently, both the gm1 and gm2 amplifiers receive feedback from terminal VO at one of their two input terminals, and each is provided with a respective input of the differential signal at respective input terminals VA and VB. More particularly, gm1 is provided with the difference in voltage between terminals VA and VO, while gm2 is provided with the difference in voltage between terminals VB and VO to form difference amplifiers.
On their output sides, gm1 and gm2 are coupled at their first outputs IP1 and IP2, respectively, to a high impedance inverting input of an op-amp OA1 and to a DC reference voltage, Vcc, through a load impedance R1. Current flows from IP1 and IP2 through R1 with the output currents from IP1 and IP2 each preferably equal to one-half of a bias current IO plus a current proportional to the difference in input voltages between the inputs of the associated amplifiers gm1 and gm2. More particularly, the current from IP1 varies with the voltage difference between one side of the differential signal at terminal VA and the feedback signal from terminal VO. The current from IP2 varies with the voltage difference between the other side of the differential signal at terminal VB and the feedback signal from terminal VO. The resulting voltage at the inverting input is indicative of the voltages between terminals VA/VO and between VB/VO, respectively.
Second outputs IN1, and IN2 of gm1 and gm2, respectively, are coupled to a high impedance noninverting input of OA1. Both IN1 and IN2 are also coupled to a DC reference voltage (VCC) through a second load impedance R2. Similar to IP1 and IP2, their output currents flow through R2 and the current for each of IN1, and IN2 varies with the difference in the input voltages on the input sides of gm1 and gm2, respectively. However, the resulting output currents for IN1, and IN2 are preferably equal to half the base line bias output current IO minus (rather than plus) a current proportional to the difference in voltages between the inputs of the associated amplifiers gm1 and gm2. The voltage at IN1, and IN2 is presented to the noninverting input for comparison to the voltage at the inverting input. Each of the outputs of gm1 and gm2 (IP1, IN1 and IN2, IP2, respectively) are characterized by equations (1) and (2),

IP=IO/2+α(VP−VN)  (1)

IN=IO/2−α(VP−VN)  (2)

where α is a gain coefficient associated with the gm1 and gm2 amplifiers. α is either a fixed gain coefficient or a variable coefficient that, for a differential transconductance amplifier implemented with an ideal bipolar differential pair, is characterized by equation (3),

α=[(IO/Vt)*exp(−Vd/Vt)]/[1+exp(−Vd/Vt)]^2  (3)

where Vt is the thermal voltage (kT/q), k is the boltzmann constant, q is electron charge, T is temperature in Kelvin, and Vd is equal to Vp−Vn. The output voltage at terminal VO settles at a voltage between the differential input voltages at terminals VA and VB in response to receiving the results of the voltage comparison between VA and VB with VO.

FIG. 2 is an implementation of the embodiment illustrated in FIG. 1, which has two NPN differential pair transistors Q1/Q2 and Q3/Q4 to implement gm1 and gm2, respectively. The differential pairs Q1/Q2 and Q3/Q4 are coupled to terminals VA and VB, respectively, at bases of Q1 and Q4, respectively, so that each differential pair receives one component of the differential signal. Bases of Q2 and Q3 are both coupled to the output of oA1 so that each differential pair receives one component of the differential signal for comparison against feedback from oA1. In the first differential pair Q1/Q2, emitters for Q1 and Q2 are coupled to a low voltage supply bus VEE through a current source I2. Similarly, emitters for Q3 and Q4 in the second differential pair are coupled to VEE through a current source I2. Emitter degeneration resistors RE can also be provided between the low voltage supply bus VEE and each of the emitters for Q1, Q2, Q3 and Q4 to reduce the gain and improve the linearity of the differential pairs Q1/Q2 and Q3/Q4.
Output collectors for Q2 and Q3 are coupled to VCC through impedance R2 so that the voltage at the noninverting input of OA1 is determined by the value of the R2 resistor multiplied by the sum of Q2 and Q3 currents. Similarly, the output collectors for Q1 and Q4 are coupled to a positive DC reference voltage through impedance R1 so that the voltage at the inverting input of OA1 is determined by the value of resistor R1 multiplied by the sum of the Q1 and Q4 currents. Preferably, the impedances R1 and R2 are equal, so that the voltages at the inverted and noninverted inputs of OA1 are the same when the sum of the Q2 and Q3 currents matches the sum of the Q1 and Q4 currents.
The current sources I1 and I2 are preferably matched so that an equal voltage differential between the bases of Q1/Q2 and those of Q3/Q4 produces voltages at the inverting and non-inverting inputs of oA1 that are substantially equal to one another. The equal voltage differentials are accomplished by oA1 driving its output at terminal VO to a voltage centered approximately between the inputs of VA and VB. Such a voltage is approximate to the average or common-mode voltage VCM so that the output at terminal VO settles at a voltage indicative of the common-mode component of the differential signals.
For a common-mode detector designed for a differential voltage across VA and VB of between −1.0 V and 1.0 V, and a common-mode voltage VCM between −1.0 V and +1.0 V, the components could the following values:

R1, R2=1 Kohm

RE=100 ohm

I1, I2=1 mA

Although the differential transconductance amplifiers are illustrated using NPN bipolar transistors, they could instead be implemented using any transconductance device, including (but not limited to) bipolar, metal oxide semiconductor (MOS) or junction field effect transistors (JFET).
FIG. 3 is a logical flow diagram illustrating a method of detecting a common-mode component in a differential signal for a common-mode detector. The method begins with differential components VA and VB provided to the detector and with an output VO from the detector (block 300). The voltage of output VO is subtracted from the voltage of each differential component VA and VB to produce respective currents ΔiA and ΔiB (blocks 305, 310) that are indicative of the voltage difference at the respective inputs to each gm amplifier. More particularly, the voltage subtraction between VA and VO results in a current ΔiA indicative of the difference in voltages for use by the detector. Similarly, the voltage comparison between VB and VO results in a current ΔiB. If ΔiA is substantially equal to ΔiB (block 315) then VO remains unchanged and the cycle starts back at the beginning (block 300). If ΔiA is greater than ΔiB (block 320) then the voltage level for output VO is moved closer to the voltage level for VA (block 325). In the embodiment illustrated in FIG. 2, a ΔiA larger than ΔiB would produce a larger voltage drop across R1 than across R2. The inverting input of the feedback amplifier would be presented with a smaller voltage than would the noninverting input, and thus the VO feedback signal would be driven toward VA. If instead, ΔiA is less than ΔiB (block 320) then the voltage level for output VO is moved closer to the voltage level for VB (block 330). The process repeats with differential signals VA and VB and newly modified output VO (block 300). In this manner, the detector uses its own voltage output VO as feedback for comparison to the differential components VA and VB.
Or, the common-mode detector can generate voltages ΔVA and ΔVB, instead of currents ΔiA and ΔiB, in response to comparing the voltages of VA and VB with the output voltage VO. The voltages ΔvA and ΔvB would be indicative of the difference in voltage between VA and VO, and VB and VO, respectively. VO would settle at a voltage level approximating the average of VA and VB, which defines the common-mode voltage VCM.
While various embodiments of the invention have been described in terms of a common-mode detector in an ATE, it will be apparent to those of ordinary skill in the art that many more embodiments and implementation are possible within the scope of this invention that are removed from an ATE, such as in other circuits requiring detection of the common-mode, or average, component of a differential signal. Accordingly, the invention should be limited only in terms of the appended claims.

## Claims

1. A common-mode detector, comprising:
a first difference amplifier that is connected to compare a first input voltage with a feedback voltage to provide first and second differential current signals;
a second difference amplifier that is connected to compare a second input voltage with said feedback voltage to provide third and fourth differential current signals;
a feedback amplifier having inverting and non-inverting inputs, said feedback amplifier connected to receive said first and fourth differential current signals at said inverting input and said second and third differential currents at said non-inverting input to drive said feedback voltage to a level that is substantially the average of said first and second input voltages in response to receiving said first, second, third and fourth differential current signals.

2. The detector of claim 1, wherein said first and second difference amplifiers comprise:
first and second differential transconductance amplifiers, respectively.

3. The detector of claim 2, wherein said first transconductance amplifier comprises:
first and second outputs coupled to said inverting and non-inverting inputs, respectively, so that said first output provides current IP and said second output provides current IN according to


IP=IO/2+α(VP−VN)



IN=IO/2−α(VP−VN)



wherein IO is a bias current, VP is said first input voltage, VN is said feedback voltage, α is a gain coefficient of said first differential transconductance amplifier and IP and IN comprise said first and second differential current signals, respectively.

4. A common-mode detector, comprising:
an op-amp having inverting and non-inverting inputs and a detector output;
first and second differential transconductance amplifiers, said differential transconductance amplifiers each having:
a first input;
a second input coupled to said detector output;
a first output coupled to said inverting input; and
a second output coupled to said non-inverting input;

wherein, in response to receiving a differential signal at said first inputs, said common-mode detector is operable to provide a common-mode detector output indicative of a common-mode component of said differential signal.

5. The detector of claim 4, wherein said first differential transconductance amplifier further comprises:
first and second transistors each having a collector coupled to said inverting and non-inverting inputs, respectively.

6. The detector of claim 5, further comprising:
a first current source coupled to emitters of said first and second transistors.

7. The detector of claim 6, further comprising:
an emitter degeneration resistor coupled between said current source and each of said emitters.

8. The detector of claim 4, further comprising:
a first impedance coupled between a reference voltage and each of said inverting input and first outputs.

9. The detector of claim 8, further comprising:
a second impedance coupled between said reference voltage and each of said noninverting input and said second outputs.

10. The detector of claim 9, wherein said first and second impedances comprise first and second resistors, respectively.

11. The detector of claim 4, wherein said first differential transconductance amplifier further comprises:
first and second transistors with their output collectors coupled to said first and second outputs, respectively, and their bases to said first and second inputs, respectively;
so that said first differential transconductance amplifier is operable to receive said detector output and a first side of said differential signal.

12. The detector of claim 11, wherein said second differential transconductance amplifier further comprises:
third and fourth transistors with their output collectors coupled to said second and first outputs, respectively, and their bases to said second and first inputs, respectively;
so that said second differential transconductance amplifier is operable to receive said detector output and a second side of said differential signal.

13. The detector of claim 12, further comprising:
a current source coupled to emitters of said third and fourth transistors.

14. A method of measuring a common-mode component of a differential signal, comprising:
converting a voltage differential between a non-inverting side of said differential signal and a feedback signal to generate a first differential current signal;
converting a voltage differential between an inverting side of said differential signal and said feedback signal to generate a second differential current signal; and
comparing said first and second differential current signals to generate said feedback signal, so that said feedback signal settles to a voltage indicative of the common-mode component of said differential signal.

15. The method of claim 14, wherein said comparing step comprises generating first and second voltages corresponding to said first and second differential current signals, and comparing said voltages to generate said feedback signal.