Problem with AD8293G160

Hi,

I have a problem with AD8293G160. (please see PDF attachment).

The Gain in my test is not 160. What is wrong?

This is my first thermocouple circuit and i selected AD8293G160 for it's very low cost.

I absolutely need a very cheap circuit for my application.

The AD8496 is much more expensive than AD8293G160.

Many Thanks for Your suggestions.

Sandro

AD8293G160Problem.pdf
  • Hello Sandro,

    To measure gain, you need to make a two-point calculation. Simply dividing the output by the input is not valid because it takes other errors into account. The correct calculation is Gain = (Vo2-Vo1) / (Vi2-Vi1) where the numerator is a voltage change in the output caused by a voltage change at the input (denominator).

    In addition, you may be injecting additional measurement errors when probing nodes like the inputs or the reference terminals. Sometimes is better to generate the signal from a reliable source and make measurements around high impedance nodes (like the reference input in your case) with a high-input impedance DMM (such as a bench-top DMM in which you can disable the 10M input impedance and the auto-zeroing function).

    Regards,

    Gustavo

  • Thank for Your answer. I already use a DDM (Fluke 8845A) and i tried with or without 10M input impedance but the results are the same. I also put a termometer (Fluke 51) with its termocouple very close to the termocouple connected to J10 in my schematic for a temperature comparison. First i read with DDM the voltage of my termocouple J (not connected to J10) . Then i connect the termocouple to J10 and i read the analog amplified value by U5 (without interference due to DDM) and display it, but it's not correct, i don't have the aspected gain. Any suggestion? Thanks.

    Best regards.

    Sandro

  • Hello Sandro,

    I wouldn't involve temperature measurements just yet. Something you can do instead is take a dc voltage source and apply it to the amplifier. Measure both input and output with two different input voltages and perform the calculation I suggested before.

    Regards,

    Gustavo

  • Hello,

    I measure both input and output with two different input voltages and perform the calculation You suggested before but the Gain is lower than 160. (from 120 to 140). I removed C25, C26, C27 and repeated the test with the same results. Then i removed also R44 and decrease the value of R31, R32 to 1K but it dosn't work properly...the Gain is absolutely not guaranteed to 160 but it change (from one board to another). The precision of resistor is 0,1%.

    Do Analog Device have a solution? A correct and tested scheme for thermocouple to implement with this device?

    If its not a good amplifier for thermocouple like reported on the first page of datasheet, which other low cost device i can use?

    Thanks

    Sandro

  • Hello Sandro,

    I am sorry to hear you are still having trouble with this. The maximum (guaranteed) gain error specification is 1%, which means you shouldn't be getting a gain less than 158.4. Guaranteed specifications such as this one are tested in production, so you either have an error in your measurement procedure/circuit or you have a defective part(s).

    Let's take a look at the first option: removing the capacitors is probably a good idea just for the purpose of testing, but you can't remove R44 and expect it to work, unless your signal source is grounded to the same point as your power supply. The inputs need to be biased inside the operating range of the amplifier inputs. R44 provides this bias. Instead, you can try to short it out.

    What did you use as a signal source for your tests? Do you have your measurement results? Are you measuring the output before or after R40?

    The only other possibility is that your reference is too close to the specified 0.8V. You should try to increase this to 1V, 2V, etc to see if that makes some improvement on the gain accuracy.

    Regards,

    Gustavo