Question asked by leilash on Sep 14, 2011
Latest reply on Sep 14, 2011 by Matt.Duff

Hello

as you can guess from the title , my problem is very simple

i want to connect a sensor with voltage output around (0 - 500uV) to adc of C8051F124 from SiLab, my problems are :

1. when i connect sensor output to IN+ and IN- to GND , giving AD620 +5V and -5V and using a near 500Ohm resistor i get gain of 1000 which i shouldn't according to datasheet instead i should get around 100 (in this stage there is only the op-amp and the sensor checking voltages using a voltmeter , circuit is very simple i just have used one resistor for gain, +,-5V for powering op-amp , GND as refrence and output of sensor is connected to IN+ and IN- is connected to GND ).

2. when i connect the output of AD620 to ADC of the microcontroller directly i get a current of near 10 ma  going through the circuit (which worries me a little because i think this current is going through microcontroller) and the output voltage of AD620 goes from 0-500mV to 1-1.5V which means there is one volt offset (the most weird part is that when i use a volt meter there is no voltage drift and the current is normal too but when i connect the ADC then current goes up to 10ma and the voltage gets 1V offset)

Now i want to know that do i need any interfacing circuit ? i mean i wasn't expecting to see that 10ma current because when i connect ADC of uC to a series of resistors for reading some voltages i do not get that much of current (i get around 200ua) but when i connect my ADC to AD620 i get this amount of current with voltage drift.

And my another question is that am i connecting the whole sensor thing and ADC right ?

Some more info : the GND of sensor , AD620 and uC are connected to each other, +5V and -5V voltages for the AD620 Vs+ & Vs- are highly stable usign voltage regulators. the capacitance of uC in input ports is around 10pf and if you need anymore information write here and i will give them to you.

Thanks for helping me