Consideration for AD8421

Hi, Chaitanya.

AD8421 is guaranteed to operate on +/-15V supply for dual supply. In your case, +30V and -2.5V exceeded this range. Eventhough the absolute maximum supply range specifies +/-18V, the performance will not be the same as the desired or expected if the abs max is used as supply range. So to get the proper operation of AD8421, try using +/-15V for dual supply and 30V for single supply.

Hope this helps!

Thanks,

Anna 

Hi, Anna

Thanks for your reply. AD8421 absolute max voltage rating is 36V,  but i have applied only 32.5V. I want the amplifier output to swing from 0V, so i have applied the negative supply. Input voltage range is from 0V to 24V, so i have applied +30V as +Vs. If i operate the amplifier with +/- 18V, common mode voltage range will be exceeded. What i have observed is at unity gain, amplifier operation is perfectly alright even with +30V and -2.5V.  What is to be done, to make the amplifier  work properly at gain 100?

Regards,

Chaitanya.

Hi Chaitanya,

It looks like you are running into the Input Common-mode vs Output Voltage Swing hexagon plot limitation. The AD8421 datasheet shows this limit for a few standard supply voltage ranges and gains in figures 10 through 13 in. It is explained in more detail in this video, but the basic idea is that the internal nodes can saturate in addition to the input and output limits, similar to if you built an instrumentation amplifier out of 3 op amps.

In order to maximize the output swing for an instrumentation amplifier like the AD8421, you typically need a common-mode voltage somewhere near the mid-point between the two supplies.

Just a couple questions to help make a recommendation. For your G=100 circuit, do you still need 0 to 24V input common-mode range? Also what is the differential signal range? Is the Reference pin grounded?

Regards,

Scott

Hi Scott,

Thanks for you reply. That video seems much helpful for me. I have increased -Vs to -4.5V, reduced +Vs to 28V. Circuit is working properly at gain 100 with the modified supply range. (i.e I have tested the circuit with 0.06V). My application is to amplify the difference between 2 inputs. One of the input is +24V and the other is +23.94V. I need the output as 6V, so i have selected gain of 100. With reference to the video, here common mode voltage=23.97V, output of first stage should be +26.97V & +20.97 for gain 100. Everything seems fine if i increase the +Vs to 29V, as the input maximum range is +Vs-1.8V. Am i correct?

Regards,

Chaitanya.

Hi Chaitanya,

I am glad the video helped! I think Matt explains it very well.

There is one more detail. For the AD8421, there is a slight modification from the standard limit. The NPN input transistors shift the voltage down by 1 Vbe. So the outputs from the first stage will actually be about 0.6V lower than you would originally think. This actually helps you get a little bit more head-room on the top side.

It looks to me like setting +Vs to 29V should be plenty. In fact, even your original +30/-2.5V supply voltage might have been fine. If you tested it with +23.94V at the negative input and 60mV between the two inputs, it should have worked okay, but if you tested it with -IN grounded and +IN at 60mV, it wouldn't have worked.

Best regards,

Scott

Hi Scott,

Yes, that video has helped me so much. I used to see the common mode voltage in different way.(i.e Vin should be always below the Vcm(hi) and Vcm(lo)). But that video has clarified me the doubts about common mode voltage. Thanks a lot for sharing the informative video. In order to ensure the gain i have checked with 60mV for calibration purpose. Now the circuit is perfectly alright, in all required conditions. I am so much impressed with your response. Once again thank you so much.

Regards,

Chaitanya.