AD637: chip select function - All outputs High Impedance?

Document created by analog-archivist Employee on Feb 23, 2016
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My question relates to the functionality of the chip select function. When CS
is tied to a low voltage (less than 0.2V) the datasheet indicates that the
output goes to high impedance. My question is whether the output of the buffer
amplifier also goes to high impedance or if this is only for the rms output of
the device.

The part is connected on the system using the provided Buffer amplifier to
implement a 2nd order Sallen-Key low pass filter, as in figure 11 on revision K
of the part datasheet.

 

I was able to connect with the AD637 designer today and he explained the
circuit operation with me. The rms core is the only circuit that is truly high
impedance, the buffer appears as an 80k resistor connected to the negative
supply. The dB output appears as a back-to-back diode connected across the
supply pins, there is no current flowing so the impedance is high, but this
would not actually considered as a high impedance output. For the application
noted below, the customer uses the buffer as the amplifier in an active filter
so I’m thinking the filter would not function as it would with the power
applied.

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