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Question asked by markus_k on Dec 6, 2017
Latest reply on Dec 12, 2017 by markus_k

Hello community,

I'm using the AD8555 and I have Troubles with the signals.

The IC is supplied with 3V single supply and I use it to measure a diff Signal at a strain gauge Bridge with 350 Ohm.

When i take the RFI filter circuit from datasheet page 28 / figure 59 and connect both Inputs to the same potential by a voltage divider with 350Ohm resistances, the voltage directly measured at the input pins at the amplifier are not the same and the VOUT signal moves around and I can also measure a voltage drop over each 4k input resistance.

Is it because of the fault detection current?

The RFI circuit consists of C0G capacitors and the voltage at output is programmed to Vcc/2. When I remove all the input filter components and the 4k by 0Ohm, it works.


Also, when I connect an additional low pass filter with 470Ohm and 15nF/C0G at VOUT and only a multimeter to measure  the output voltage, I can measure a certain voltage drop over the resistance. When I remove the capacitor it works.


I don't have the possibility to change the firmware which is programming the AD8555. I have also tried more than one AD8555 and they all behave like my description.


Thanks in advance,