AD8429 offset voltage

Hello,

in my circuit I have a problem, with the offset voltage. I dont how is the best way to solve it.

Below you can see my circuit.

I have measurement the volteges Up, Un, Uref and Uout with different values of Rg. The relay are reed relay. My aim was it, to make a offset compensation  with the Uref voltage. But if the gain (small Rg) is to high, than I get an offset voltage, which is bigger I have calculated. Thats the reason I measurment the voltages to find out, where the big offset come from. For this measurement the Uref voltage is constant.

v Rg Up Un Uref Uout
1 10M 123,5mV 126mV -4,1mV -6,2mV
10 666,6 123,4mV 125,8mV -4,09mV -29mV
100 60,6 123mV 175mV -4,09mV -250mV
circa 1000 6 123mV 139mV -4,09mV -2,3V
circa 2000 3 124,7mV 140mV -4,1mV -3V

1.The output voltage is: (Up - Un) * v + Uref. For the first and the second line thats ok. But the other lines it is funny.

2. I understand the voltage Up, the bias current is constant and flow throw the 1Meg resistance and create the offset voltage of circa 123mV.

3. But I dont understand, why the voltage Un is change from v=100. Is this normal for the AD8429 ?

4. Have you any idea, where problem should be? Is it a measuremnt error?

Thank you in advance.

Best regards

Dirk

Parents
  • 0
    •  Analog Employees 
    on Mar 25, 2015 1:09 AM

    Hello Dirk,

    I'm glad the AD8421 solved your problem. It seems like a much better fit.

    For most in-amps, including the AD8429, large differential input voltages that overload the amplifier in high gains can cause high current flow in Rg. This is because when the preamplifier is overloaded, there is a diode connection from the inputs to the Rg pins, so your Rg resistor will see the differential input voltage minus a couple diode drops. In many cases, this can damage the resistor or the in-amp if the user is not careful, hence the recommendation that you posted from the AD8429 datasheet.

    However, one big feature of the AD8421 is the input over-voltage protection (OVP). The basic mechanism of this OVP is that it limits the current flowing into the inputs to a safe level for the in-amp, even with high gains and large differential input voltages. For example, here is the input current vs input voltage at a gain of 100 with the other input grounded:

    7af9fb215252f76901252cbc02122f2c.html

    In your case, if the input current is limited to is less than 30mA when overloaded, then there can be up to about 9mW in your 10 ohm resistor. And maybe even less than that if there is some voltage drop across your Rprotect resistors. When the signal is in range, on the other hand, the inputs are very high impedance.

    The AD8421 was designed to allow higher input current in order to improve the noise performance. We have a few low power in-amps with OVP that limit the input current even more: AD8422, AD8226, and AD8227.

    I hope this helps.

    Best regards,
    Scott

Reply
  • 0
    •  Analog Employees 
    on Mar 25, 2015 1:09 AM

    Hello Dirk,

    I'm glad the AD8421 solved your problem. It seems like a much better fit.

    For most in-amps, including the AD8429, large differential input voltages that overload the amplifier in high gains can cause high current flow in Rg. This is because when the preamplifier is overloaded, there is a diode connection from the inputs to the Rg pins, so your Rg resistor will see the differential input voltage minus a couple diode drops. In many cases, this can damage the resistor or the in-amp if the user is not careful, hence the recommendation that you posted from the AD8429 datasheet.

    However, one big feature of the AD8421 is the input over-voltage protection (OVP). The basic mechanism of this OVP is that it limits the current flowing into the inputs to a safe level for the in-amp, even with high gains and large differential input voltages. For example, here is the input current vs input voltage at a gain of 100 with the other input grounded:

    7af9fb215252f76901252cbc02122f2c.html

    In your case, if the input current is limited to is less than 30mA when overloaded, then there can be up to about 9mW in your 10 ohm resistor. And maybe even less than that if there is some voltage drop across your Rprotect resistors. When the signal is in range, on the other hand, the inputs are very high impedance.

    The AD8421 was designed to allow higher input current in order to improve the noise performance. We have a few low power in-amps with OVP that limit the input current even more: AD8422, AD8226, and AD8227.

    I hope this helps.

    Best regards,
    Scott

Children
No Data