Signal conditioning
common-mode gain to differential gain and both of these quantities are reduced by a factor of 60, the resulting CMRR will be the same for the AD8479 funnel amplifier circuit, which is 90dB for the B-grade model. One application would be measuring
voltage and current for an ac electric motor. Since ac lines are hundreds of voltages, performing accurate current and voltage monitoring can be difficult. Since the AD8479 is capable of working at these voltages, the current can be measured through the motor using a shunt resistor. Using the circuit described above, the voltage across the motor can be directly measured, therefore achieving an accurate power monitor solution with minimal effor t. Although the AD8479 is a fixed unity-
Figure 8. AD8479: Gain of 1/60 high voltage impedance measurement.
the B-grade model. The offset of the buffer is actually divided down by 60 due to the dc gain from the Ref– of the AD8479 to its output therefore the offset of the AD8479 itself is the main contributor of offset.
The resulting maximum offset of this circuit is ±17µV. Likewise, since the AD8479 op amp is no longer in a dc noise gain of 60, the CMRR errors from the AD8479 are also not gained up by 60. Since CMRR is the ratio of
INS-JUL18-LEE DICKENS BRITISH FLAG:Layout 1 21/02/2019 12:09 Page 1
gain amplifier, a precision funnel amplifier can still be achieved. The funnel amplifier can be used for many applications, including supplementing a high voltage current measurement with the associated voltage across the load. Although the bandwidth of the funnel amplifier limits the input voltage range, typical line frequencies are well within the input voltage limiting frequencies, therefore the circuit performance is ideal for these types of measurements.
Analog Devices
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BRITISH DESIGN, BRITISH MADE BRITISH QUALITY
Instrumentation Monthly August 2022
55
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