Signal conditioning
measurement at these voltages can be very difficult and expensive. However, the AD8479 is able to do so with ease. As mentioned in the AD8479 data sheet, the resistor network attenuates very large common-mode voltage down by a factor of 60 while providing a unity differential gain. There are, however, many applications that could benefit from a funnel amplifier that is able to withstand those very high voltages while measuring very large signals down to a more usable voltage domain. By leveraging the precision resistors in the AD8479, the built-in attenuation factor can be exploited to enable such a measurement. Since the AD8479 attenuates a signal down
M
by a factor of 60, the op amp inside the device must then amplify that differential signal by a factor of 60 to achieve unity differential gain. The gain is realised through the ratio of the resistors connected to the negative reference (Ref–) pin and to the output. Since the goal here is to achieve only attenuation, the gain can be bypassed by feeding the output signal back to the Ref– pin. In this configuration, unity gain is no longer attained, and a precision funnel amplifier is achieved. Since the AD8479 is in a fixed gain configuration, the amplifier is likely compensated appropriately and therefore may not be unity-gain stable. To maintain stability, one design requirement here was to ensure the amplifier is in its originally intended gain before the amplifier’s gain rolls off. The AD8479 data sheet lists the typical bandwidth as 310kHz, therefore the negative reference feedback should be rolled off before this frequency. By connecting the AD8479 output through a low-pass filter, buffering the output of the filter, and routing the buffer output back to the negative reference pin, the AD8479 can be used as a very high voltage, precision funnel amplifier.
Figure 2 (above). AD8479: Gain of 1/60 schematic.
any applications demand differential measurements in the presence of large common-mode voltages, some upwards of a few hundred volts. Precision
Figure 3 (above). AD8479: Gain of 1/60 improved block diagram.
Figure 4 (below). AD8479: Gain of 1/60 oscilloscope capture of input and output signals.
Figure 1. AD8479: Gain of 1/60 block diagram. For a precision signal chain, keeping noise
and offset to a minimum is impor tant. In order to maintain this requirement, a buffer with low noise and low offset is desirable. The ADA4522 op amp in a unity-gain buffer configuration was selected for these reasons, along with its wide supply range. This enables the ADA4522 to be powered by the same supplies as the AD8479, which reduces
52 August 2022 Instrumentation Monthly
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