AnsweredAssumed Answered

problem with AD637 Chip select

Question asked by ednqn on Oct 10, 2014
Latest reply on Dec 16, 2014 by jstaley

Hi, I'm using AD637BR with a circuit that controls the chip select pin. I implemented a 2 pole filter using the internal buffer (pins 1 and 16) that follows the RMSOUT (pin 11).

I found out that, when chip is enabled, everything works fine but when chip is disabled by driving low pin 5, I get -1,5V at buffout (pin16) and +68mV at RMSOUT pin 11.

Voltage at pin 11 can be driven to 0V by placing a pulldown resistor in it but this doesn't change buffout output.

Placing a pulldown of 10K in buffout gives me -1.53V, while 750R give me about -0.4V and 330R take it to 0.2V. No pulldown resistor make this pin float to -7.5V.

Apparently disabling the chip disables the RMS circuit but the buffer does not change to high Z output, it changes to a strange state. Its not a buffer, its not high Z, and it drives current.  For my application I need to drive Voltage output at buffout to 0V. Is there any way to do this? A low resistance pull down wont work for this.

By the way,  datasheet does not explain this and should be reviewed.

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