Differential-Amplifier Common-Mode Rejection Measurements, Part 2: Differential Amplifier CM Principles

The Innovatia Floating Differential Source (FDS) was designed to measure diff-amp differential and CM gains for voltage or current-input diff-amps. Two variable sources are connected in series, as shown below. The grounded source, VCM (VCM) supplies the CM voltage and the floating source, VDM (VDM) the differential or amplifier input voltage. In the case of high-side current sense circuits or differential transresistance amplifiers, the grounded current source (which outputs only positive port current – that is, it sinks current), IOUT (IOUT) can be used as the amplifier input. VDM can supply the current sunk by IOUT.

The floating source drives the (differential) input of the diff-amp under test (DUT), and is configured as shown on the left in voltage mode. For amplifiers with current inputs, such as high-side current sense diff-amps, an additional current sink is provided in current mode. OUT+ and OUT– are the vI+ and vI- diff-amp inputs. For different vI (VDM), vCM (VCM) can be varied over a range and vO of the diff-amp measured. Then CMRR can be calculated as a function of vI .

The FDS has a typical specification of its own common-mode rejection of the DM Source by the CM source: CMRR = Δ VDM/CMRR VCM is typically about –80 dBV or about 1/10,000. A 1 V change in VCM typically causes a 100 μV change in VDM. This is comparable to the ACM of a good op-amp, and if the CMRR parallel formula given above is applied, FDS ACM can be compensated by measuring the ACM of the DUT, then considering the FDS to be cascaded with it, divide the resulting DUT ACM by the FDS ACM . For more accurate measurements, ΔVCM/ΔVDM can be measured with an accurate DMM for the particular FDS used in testing. The FDS CMRR is not, however, constant over the range of VCM and can vary by as much as a decade.

The FDS can be used in automated tests by sending it commands through the serial port. A range of VCM voltages can be scanned while DUT output voltages are measured. The VCM range can be scanned for each value of VDM in its range, resulting in a 2D plot of CMRR versus VCM, the input-referred amplitude. Amplifier linearity as a function of CM voltage can be determined from the diff-amp output voltage as a function of vI and vCM , or vO (vI , vCM ).

High-Side Current-Sense Diff-Amp Measurements

High-side current sense circuits can be measured in current mode. The diff-amp voltage gain, Av , and its ACM can be measured as described previously. When Av is known, then IOUT is set to drop a given voltage across sense resistor, RS . If some of IOUT is diff-amp input current of the input connected to IOUT, then the input current will cause an error in the voltage drop across RS that is amplified by the diff-amp. Given the diff-amp Av , the current error can be calculated.

High-side sense circuits ideally are an open circuit to ground so that none of the high-side current is shunted through the diff-amp. The resistance of the circuit can be measured using the FDS by varying VCM and measuring the change in IOUT that keeps vO constant. Although IOUT is a current source, by varying it as a dependent variable of vO under computer control, it effectively becomes a sense circuit.

The FDS is not fully an SMU in that neither source has port current-sense circuits. (The two-port analyzer, TPA204, is planned to be the FDS functional extension, but with more circuits and at higher cost.) However, by varying IOUT based on the value of another DUT variable, it can effectively become a current sense circuit. By designing an instrument that is somewhat less than a full TPA, various measurements can be performed and DUT parameters determined while saving money on instruments with a wider range of functions. To obtain an FDS manual, see the Innovatia website The next two articles will explain some of the circuitry in the FDS and its functional interface and specifications.

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