What is the Common mode voltage range of ADA4522-2 ?

Hi Ashok,

The input voltage range of the ADA4522  for supply voltage of +/-5V is only from -5V to 3.5V.

This part  needs an input  headroom of 1.5V from the positive supply rail for it to have a normal operation.

If you violate this spec (input voltage range), the output of the amplifier will become unpredictable and can go anywhere between the supply rails.

Why do you choose the ADA4522? What is your applications?

ADA4522 is our latest zero drift amplifier but it is not a rail to rail input amplifier.

We have the ADA4528 which is also a zero drift amplifier and has a rail to rail input/output but the supplies can only goes up to 5.5V.

Perhaps if you could tell me more about your applications then we could recommend more suitable parts for you.

Best regards,

Emman

Hi Ashok,

Thank you for this information.

When you say you are encountering issue in precision, do you mean you have a target accuracy that doesn't met or the circuit  doesn't functioning well?

If you are using the uA747 with supply voltage of +/-5V and input voltage of 5V, I would expect that your circuit will not work.

Clearly, based on your conditions, you really need a rail to rail input/output amplifier for it to work properly.

I would recommend to use one of our rail to rail input/output amplifier which is the ADA4091-2.

This was also used in some of our reference circuit for this kind of application which you can see here.

Let me know if you need anything else.

Best regards,

Emman

Hi Ashok,

Rail to rail output amplifier means that output of the amplifier can go almost equal to the supply rails (of course there will be few millivolts drops to the Vce or Vds of the output transistor). But this is not necessarily mean that this kind of amplifier is also a rail to rail input.

It is useful in achieving the maximum output signal swing with low supply voltage which results in increase of dynamic range.

Rail to rail input, on the other hand, means that the input can span the entire supply voltage range.

You can easily check it on the spec table and look for the input voltage range.

This is for the ADA4522:You can see that the input voltage needs 1.5V headroom from the positive supply rail.

This is for the AD8638: it need 2V headroom from the supply rail. This is also the reason why you only see 3V on the spice model.

Since your circuit is a buffer and your input voltage can reach up to the same level as your positive supply rail, then you may need an amplifier that is rail to rail input and rail to rail output. .

Now there are amplifiers that are both rail to rail input and rail to rail output like the ADA4091, ADA4084, etc.

But for the zero drift amplifier portfolio which are well known for ultra low offset and drift, only the ADA4528 is  rail to rail input and rail to rail output amplifier but it can only go supply voltage of up to 5.5V.

There are also  only 3 Zero Drift amplifier that can accommodate supply voltage of =>+/-5V but there are not rail to rail input.

What is your target accuracy specs in terms of offset and drift?

Also what is your minimum input voltage?

Are you expecting negative input voltage?

Because if not, then you can probably use the ADA4528 with V+ tied to 5V and V- tied to ground.

I hope this helps.

Best regards,

Emman

Hi Emman,

Thanks for the reply. 

Actually my supply voltage would be a ±5V. This is a constant current circuit. You can use the below link for the circuit. This circuit was facing some issues on the precision and so on. So i thought of replacing the Op-amp(previously i was using uA747) with a better precision one. I am using a regulator to stabilize my input. Currently the input is ±15V. I am regulating it to ±5V. This is the reason i said my supply voltage would be ±5V.

Moreover, from the circuit in the link, I am planning to use a buffer at the output for measuring the output voltage. So i needed a precise, low offset opamp. I came across Op-amps and finally selected ADA4522-2,since i am using the same IC for both the Op-Amps.

I saw ADA4528, but in this Op-Amp the supply voltage is only from  ±1 to  ±2.75 which cannot be used for my application. 

 Please let me know whether i went wrong anywhere.

https://www.physics.byu.edu/faculty/petersonb/phys240/ConstantCurrentSource.pdf 

Hi Emman,

It is functioning well but as temperature changes the drift of output current is also large. So i was looking for an IC which had low offset voltage and drift. Then i came across AD8638, AD4522-2 etc. One doubt which i have is that, in the datasheets of AD8638 and AD4522-2 it is mentioned that it is a rail to rail output Op-Amp, but you said above that there is a headroom of 1.5V. I tried simulating the spice model of AD8638 in LT spice. The positive output is getting saturated at 2.9 V. 

How can we come to know whether a given Op-Amp is rail to rail or not? Please provide me a clear understanding on this. 

Is the datasheet wrong? Where in the datasheet it is mentioned about the headroom voltage? You were telling about 1.5V headroom on ADA4522-2. I did not find this anywhere. Please help me on this also..

Thanks and Regards,

Ashok

Hi Ashok,

I would follow harry's recommendation regarding the C4 as well as the tolerance and tempco of the R3 since it directly affects the accuracy of your current source.

I checked the AD8572 and it is also up to 5.5V supply voltage only. Its newer version is the one that I suggest to you which is the ADA4528.

There is no so much pros and cons for deciding of having a dual or single supply voltage in terms of performance.

Note that the opamps doesn't have a ground pin.  It doesn't  know where the ground is. It only knows where each pin is in relation to other pins. Therefore it doesn't care if you are using single, dual,  symmetric or asymmetric supply as long as the you keep the inputs and outputs within the specified input/output voltage range.

Best regards,

Emman

Hi Emman, 

Thanks for the reply. It did provide me good information.

I was looking for a dual op-amp with ±5V. So i feel we cannot use AD4528. Instead i have found AD8572 as a better option for the current scenario. 

i am targeting for an offset of less than 10uV, so that no matter the temperature changes i would still be getting a stable output.

As of now I dont need negative voltage, because my output will be always positive. 

I was just following the legacy circuit. 

Since you asked me this, I would like to know what is the pros and cons of selecting single and dual Op-amps. Is it only depending on whether the application need positive and negative voltage??

What difference does it makes in a buffer circuit when I am using  ±5V and +5V and ground as the supply voltage??

Regards,

Ashok