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.