Question
1. He used 2 AD652 as V-F Converter as the below figure shown, he configured
the upper AD652 as the "Positive Input Voltage with Dual Supply", and the
bottom AD652 as the "Negative Voltage Input", and he connected the 7th pin of
the upper AD652 to the 6th pin of the bottom AD652. Then he input a signal to
the two AD652. He counted the two chips' output pulse, and observe whether the
number of the output pulse is same. When he applied a DC voltage at the two
AD652's input, the counts for two chips are same, but when he applied 5 Hz( +-
8V )AC signal to the input, he found the counts for two chips are different.
I think his application is wrong, when the AD652 is configured as "Negative
Voltage Input", the input voltage must be negative, isn't it? Similarly, when
the AD652 is configured as "Positive Input Voltage with Dual Supply", the input
voltage must be positive, isn't it?
2. Whether the input voltage signal can be AC signal?
3. In the 9th page of the datasheet, the Section "SVFC CONNECTIONS FOR NEGATIVE
INPUT VOLTAGES" said: "In this configuration, the input is a high impedance,
and only the 20 nA (typical) input bias current of the op amp must be supplied
by the input signal." I am not sure the meaning of the sentence, I think the
bias current of the amp is the error of the Amp, it shouldn't be supplied by
the input signal. Would you please tell me details about the sentence?
Answer
when the device is configured for a positive analog input, the signal must be
positive and when configured for a negative analog input, the signal must be
negative. So, the signal applied to pin 7 of device 1 needs to be inverted
before being applied to pin 6 of device 2.