Trouble with MAX 4208 Instrumentation amplifier

Hi,
I am using MAX4208 in Wheatstone bridge configuration as per attached image.
The supply voltage for bridge is 2.7 V and MAX4208 voltage is 5V. The resistance of Detector and Compensator is 3 ohm each and 1 k are on other side of bridge.
When Grounds of 2.7 V and 5 V are isolated (Driving sensor and MAX4208 from different power sources) the circuit works as expected. But when both power supply Ground terminals are shorted together the output of MAX4208 starts fluctuating randomly.
Also when we changed resistance at L1 and R1 to 5K and resistance at L2 and R8 to 100 Ohms the MAX 4208 works properly even if we short Ground terminals of power sources together.
In our case we have single power source of 5 V and from 5 V supply 2.7 V for sensor has been derived. So we can't have isolated Ground terminals. Also the resistance value of L1 and L2 are 3 ohms each.
Using this configuration I am getting lots of fluctuations in output of Max 4208.
Kindly advice how to go ahead with this.
Thanks,
Paddy
View attachment MAX4208_Test_Conditions.pdf

Hi,

please clarify:
* L1 and L2 values: is it 3Ohms (text), 1k (text, schematic), 5k (text), 100R (text, schematic), 200R (schematic)?
* what is "+VCC"?
* What is +VCC referenced to? (which GND?)
* Why is gain= 11, and sometimes gain = 16?
* Z1 is not assembled?
* sometimes it is "+5V" sometimes it is "5V"
* what is this "5V" referenced to?
* where does "2.7V" come from?
* What is is "2.7V" referenced to?

***
Mind: For good performance, the 2.7V need to be as precise as VRef.

Measure: In case it is not operating as expected:
* what is 2.7V exactely?
* Is it fluctuating?
* what´s it´s current?

Klaus

Hi Klaus,

L1 and L2 are sensors whose resistance is 3 ohm each. When the parameter (physical entity which is to be measured) is detected L2 resistance decreases by some value and bridge will be imbalanced. That difference signal is to be amplified by MAX4208.
Whatever values mentioned in schematic and test are variable values that I have used during testing. Please ignore those values and refer below mentioned values.

L1 and L2 sensor resistance- 3 ohm each
R1 and R8- It can be anything from 100 ohm to 1 k (Balance the Wheatstone bridge)
Wheatstone bridge supply voltage is 2.7 V. That 2.7 V is from voltage regulator IC.
The circuit operates on 24V DC Battery and Buck converter is used for 24V to 5V conversion. And voltage regulator IC (TL 5209) is used for 5V to 2.7V conversion.

During testing of MAX 4208 I have figure out one problem. I am testing only MAX 4208 IC without Wheatstone bridge circuit. Supply voltage to MAX 4208 is 5V.
Two separate voltage sources are used to give input to positive and negative input terminal. Negative terminals of both voltage sources are shorted together.
The IC works fine till input voltages are up to 500 mV and less.When Input voltage goes above 500 mV the output of MAX 4208 starts fluctuating abruptly.
I have maintained the voltage difference between common mode voltage input is 50 mV. Till 500 mV it works fine but above 500 mV it is not working.
The sensor voltage in our case is 2.7V. So common mode voltage is 1.35V.
Please suggest if I am doing anything incorrect.

Thanks
Paddy

Hi,

Still a lot of information missing.
--> give complete schematic, including power supply, for the non_working circuit.

3Ohms is a rather low ohmic sensor:
* what is the expected change in resistance?
* you say other resistors of your bridge you can select...for a good thermal stability all four resistors should be made from same material on the same substrate to ensure same temperature.

Two 3Ohms resistors in series, supplied with 2.7V means about 0.5A....and 1.2W of power dissipation. Not really meaningfull values for a sensor.

And again: give the requested measurement values. Save our spare time

Klaus

Hi,

Sorry for delayed reply.
Please ignore about Wheatstone bridge and resistance values of sensor for now.

I have given direct voltage to input terminals through two separate voltage sources instead of bridge.

The configuration is
MAX 4208 is powered through 5V DC single supply.
Input at pin no 2 is from one voltage source. (Input given 100mV to 1V)
Input at pin no 3 is from another voltage source. (Input given 150mV to 1.05V)
Common mode voltage difference is 50mV.
All Ground terminals are tied together.
Gain is set to 10.

But the results are not as expected of MAX 4208 when Common mode voltage at input pins (Pin No 2 and 3) is above 500mV. When Common mode voltage is less than 500mV it is working fine. But after 500mV it starts fluctuating abruptly.

Can you please advice why this is happening?

Thanks
Paddy

I don't like the test setup with two voltage sources because output fluctuations of voltage sources can cause unexpected results. Better use separate common mode and differential mode (the latter can be shorted for pure common mode test) sources.

How did you bias REFIN in the test?