Re: electrical problems with Galvanic Skin Response circuit ( GSR SCL )

Hi Stefan,

Maybe i found the reason for the behaviour of the AD623, that the 
outputvoltage was always to little.
In the datasheet of the AD623 you can see a limitation of the 
outputvoltage depending on the common-input mode.
In my understanding we deal with voltages around 500mV

Vin- : 500mV
Vin+: 600mV

So the common-mode input voltage is (500mV+600mV) / 2= 550mV.
According to the diagram the maximum output Voltage then is about 2,2V.

To that i have two questions:

1. Do i have to set another opamp after the AD623 to get a full range of 
0-5V for the AD-converter?
Do i have to make sure, that the output voltage is always lower than the 
max. Voltage from the diagram?

2. Or do i have to make sure the common-mode input is exactly 1,8V, to 
get max. 5V Output Voltage belong to the diagram?
But is this possible with 500mV Vref? The changing skin resistance would 
change the common-mode inut voltage too, isnt it?

But how is it possible that i got 4,18V at the output from AD623 with 
0,5V at Vin- and 1,55V at Vin+?
The common-mode input voltage is (500mV+1550mV) / 2= 1025mV.
According to the diagram the maximum output Voltage should be smaller 
than 3,1V.

Or do i misunderstand the diagram.

Regards Steffen

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