problem with AD637 Chip select

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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  • Thanks Kris: I've built my own pcb. Below you can see part of the schematic. AC signal comes from a band pass filter. In J41 I can place a configuration jumper that conects pin 5 to a LM319 output that drives this line to GND when the comparator changes its output in that sense. Jp4 and TP13 are used for fine trimming as described in the datasheet. In adition, I've placed P25 for scale factor trimming, and implemented a 2 order filter with internal buffer for ripple smoothing. Circuit accuracy is pretty good. As stated before, when disabled, voltage in pin 16 is -7,5V and changing R150 value changes this voltage. Also, I've measured +68mV in pin 11. This voltage can be driven to 0V placing any resistor between pin 11 and gnd. I expected that driving RMSOUT to 0V, would drive BUFIN to 0V and then BUFFOUT to 0V too, but I found that BUFFOUT is forced to -VS when chip is disabled.

    Apparently there is no way to solve this. internal buffer should not be used if CS pin is not tied to +VS.

    Hope this helps. Thanks in advance.

    ED.

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  • Thanks Kris: I've built my own pcb. Below you can see part of the schematic. AC signal comes from a band pass filter. In J41 I can place a configuration jumper that conects pin 5 to a LM319 output that drives this line to GND when the comparator changes its output in that sense. Jp4 and TP13 are used for fine trimming as described in the datasheet. In adition, I've placed P25 for scale factor trimming, and implemented a 2 order filter with internal buffer for ripple smoothing. Circuit accuracy is pretty good. As stated before, when disabled, voltage in pin 16 is -7,5V and changing R150 value changes this voltage. Also, I've measured +68mV in pin 11. This voltage can be driven to 0V placing any resistor between pin 11 and gnd. I expected that driving RMSOUT to 0V, would drive BUFIN to 0V and then BUFFOUT to 0V too, but I found that BUFFOUT is forced to -VS when chip is disabled.

    Apparently there is no way to solve this. internal buffer should not be used if CS pin is not tied to +VS.

    Hope this helps. Thanks in advance.

    ED.

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