Hi
I use AD421 in alarm-current-operation mode only (17bit mode) and I am facing a strange behaviour of AD421. Time to time AD421 jumps from correct current output to aproximately double - e.g. jumps from 11.xx mA to 22.xx mA. It seems to be an extra clock that leads to data shift by one bit to double values. I have checked it using osciloscope and logic analyzer, but there are not any extra clocks and the output looks like:
D2,D1,D0 (data, clock, latch) - SPI
yellow - current (sensing resistor 100R)
As you can see the data are the same but the current output changes (to double the value). This happens irregularly within minutes (refresh data rate is 1 s).
There are optocouplers between MCU and AD421 therefore I tried to place buffers with Schmitt-Trigger inputs (type SN74LVC3G17). Unfortunatelly without effect.
The best solution is to place 10p capacitors between Vcc and CLOCK and between CLOCK and COM. It improves the stability to one spike per 16 hours but I know it is opossite to suggestions in datasheet to keep the rise and fall times as fast as possible. How is it possible ? What causes such behaviour ?
Martin
