Question
Using AD8552 to build a zero offset InAmp
Answer
The circuit you describe using 3 AD855X opamps will not work. The gain is too
high and even with a common mode voltage equal to mid-supply, the first stage
amplifiers will saturate when the input is greater than 7mV.
The maximum common mode input voltage is a function of the input voltage and
the gain chosen. It's not a simple calculation to make. The AD623 datasheet
discusses this problem in detail. What you have to do is analyse the circuit
for each possible combination of input voltage, gain and common mode and work
out the voltages at the output nodes of the Amplifiers A1 and A2 and the at
the input nodes of A3. You will then end up with a graph similar to figures 21
and 22 in the datasheet for the AD623. Note that the AD623 uses level shifting
transistors on the input nodes to go to ground (not the same as 0V common mode
voltage).
The enclosed file (Produced in PSPICE) shows the node voltages for a 5mV input,
2.5V common mode voltage and a gain of 167.
The output voltage on A1 is given by Vcm + (Vin x Gain). The output voltage on
A2 is given Vcm - (Vin x Gain). Therefore for a 10mV input voltage, a gain of
350 and a common mode voltage of 2.5V, the voltage at the output of A1 would
require to go to 6V and the voltage at the output of A2 would be required to go
to -1V which is beyond the output range of the AD8552.
Further Application notes and technical articles can be found on our website:
www.analog.com/InAmps
The brief analysis above shows that the output nodes of A1 and A2 will saturate
even with a 2.5V common mode voltage. With a 2.5V common mode, gain of 350, the
maximum input range would be 7mV. As the common mode voltage is increased or
decreased, the maximum input voltage before saturation will decrease. You could
reduce the gain, reduce the input voltage or use an amplifier with dual
supplies in order to get a satisfactory design.
When designing an instrumentation amplifier, it is important to note that
offset voltage is only one parameter which will limit the accuracy of your
design. Other parameters to consider are, input impedance, noise, common mode
rejection, power supply rejection and temperature drift. The app notes included
on the website will give you greater insight into designing InAmps and issues
involved. Resistor matching is critical for good CMRR performance. On a silicon
chip matched resistors which track with temperature are relatively easy to
make. To get discrete matched resistors is generally expansive. The matching
and drift of the external resistors is likely to be a greater source of error
than the offset voltage.
Looking at your application, it looks like you want to interface to a
thermocouple which has an output voltage of 10mV. If you ultimately need to
digitise the signal, have you considered the AD7705? This sigma delta converter
will take a 10mV signal directly and digitise to about 12 or 13 bits of
precision. If you are using a thermocouple, you also need to perform cold
junction compensation. You could use the second channel on the AD7705 connected
to a silicon temperature sensor such as the AD590 to measure the temperature at
the cold junction and perform the compensation in software.