AD977A: Driving the inputs

Document created by analog-archivist Employee on Feb 23, 2016
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For our project, our pcb design is completed by using ad977a. After that point
we started pcb tests. We observed problems for digitization process. We are
trying to digitize the analog signal which is +/-10 v and 1k ohm output
impedance. Can you please say that 1k ohm output impedance is a problem.


The AD977A uses an on-chip resistive divider to scale and shift the +/-10V
input range so that it fits the 0 to 2.5V range of the ADC. This resistive
scaling circuitry has a realtively low input impedance, table 1 on page 14 of
the AD977A datasheet shows that the AD977A input impedance is 11.5kOhms for the
+/-10V input range. This must be driven from a very low impedance source if
errors at the 16-bit level are to be avoided. In practise the best way to
achieve a very low source impedance is to buffer the input. We have an op amp
whch is especially suitable for buffering the input to a 16-bit ADC, the
AD8021. The datasheet for the AD7663 shows how to use the AD8021.