Re: AD8221 Problem

Hi Matt, ^_^~

I also want to ask questions about AD8221 here. Actually I built a very simple circuit with it, just as Figure 47 in AD8221 Datasheet (rev.C). I use 500ohm as Rg, +-15V as Power Supply, with 0.1uF capacitor on each power supply pin to REF, REF is connected to ground, load is 10kohm. I just wanted to test this circuit but encountered problems.

My input is generated by a TEKRONIX Signal Generator, and its negative polar is connected to ground. Input range is 10mV to 100mV, f is DC to 1kHz. But no matter which signal I tried, the output stayed on zero all the time! No Output...By the way, I use non-isolation oscilliscope to observe the curve, so -IN and REF can be viewed as tied together. I really don't know the I need some special measure instruments, like isolation probe, to observe circuits with AD8221 ?

Another question is, must the REF to be tied with the middle point of the Power Supply (I mean the 0V point) ? If not, what is the range of their difference? REF can be maximum as +Vs and Minimum as -Vs ?

Hope you can help me with that.

- Halle

  • 0
    •  Analog Employees 
    on Apr 19, 2012 5:27 AM

    Hi Halle,

    Matt recently moved to a different role within the company, but hopefully I can help you. Your circuit sounds fine if I'm picturing it in my head right, but would you mind drawing it for me to make sure we're on the same page? It doesn't typically take a special kind of probe to measure the AD8221 output. However, we usually use 10x attenuating probes just to avoid much capacitive loading.

    A few sanity checks: What is the power supply current? Does anything change when you remove the load? Did you check the time base of the oscilloscope? Also check the inputs and ref just to make sure nothing unexpected is happening.

    To address your ref question, Vref can be set anywhere within the supplies as long as it's set with a low impedance source. However, the output can still only swing within a bit more than a volt from the rail. Also there's the diamond plot limitation shown in figures 8 and 9 of the data sheet. Vref affects the shape of this graph as well.

    I hope this helps.


  • Hi Scott!

    Thank you very much for help!

    My circuit is as below, I even draw the measure instrument = =#...I have tried several times today but the problem remains, or even more.

    (the simplest one..)

    I have checked the circuit as you suggested:

    The power supply current: when +15V it is 2.13mA, when -15V it is 3.6mA;

    There is no change when remove or add the load;

    Time base of oscilliscope is fine;

    I have connected the 0V of Power Supply with GND of Oscilliscope and the GND with Signal Generator, use extra wires, to provide a stable REF for signals and power. Input is correct and can be measured by probes between pin 1 and 4 of my AD8221. Power Supply works well. (all these methods are the same with yesterday's)

    But today's output remains on +15V(DC) when my input is no matter 10mVpp or 20mVpp 10kHz sine wave (offset = 0)...I guess my original "no output" maybe the probe has some problems, because I changed with a new one and it can tell the output as "+15V"....although this is still wrong.

    How can I input an AC signal but output a DC one? and I doubt that whether AD8221 can be used to amplifier not-differential signals (my -IN is actually REF)? and if so, maybe it's because my signal generator can not generate a differential signal ?

    Thanks for your time and help again! Thanks for your patience~


  • 0
    •  Analog Employees 
    on Apr 20, 2012 5:43 PM

    Hi Halle,

    Something doesn't add up here. The quiescent current of the AD8221 should be about 1mA. The output shouldn't be able to go all the way to 15V, but if it was close enough that it looks like 15 on an oscilloscope, there would be an extra 1.5mA through the 10k resistor. Except that 1.5mA would have to come from the positive supply, so I would expect about 2.5mA from +Vs and 1mA to -Vs. Follow your wires carefully and make sure they're hooked up to where they're supposed to be. Make sure -Vs goes to pin 5 and +Vs to pin 8, the pin 1 dot is where you expect it to be, and no pins are shorted together by solder blobs. If all of that is right, I'm afraid your AD8221 may be damaged.

    A couple other tips: Usually when testing an op-amp or an in-amp like this, it's a good idea to place a large value resistor between the positive input and ground, that way the bias current can flow when you don't have an input signal, which prevents the part from railing at turn-on. You may do this already, but always be careful to turn your supply on before your input signal and turn off your input signal before your supply. That way you don't have voltages beyond the supply rails, which could turn on the ESD protection diodes in the amplifier.