I would like to know the common mode voltage range of ADA4522-2. I am using this as a buffer. The supply voltage to the Op-Amp is ±5 V and my input would be near to 5V( assume 4.9V). What can be my possible output value.?
Thanks in advance.
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.
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.
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.
C4 is BAD!!! Remove it!! If the op amp is a perfect voltage source, then the cap does nothing.
But op amps have a finite output impedance, so you create an R-C low pass which
gives phase shift and reduces stability, leading to oscillations.
I would guess that the majority of your drift over temp comes from the resistors, especially R3. Is it a precision
resistor with a guaranteed tempco of less than 5 ppm/C??
I am changing the resistors also to 0.1% tolerance with 25ppm/C. I feel this would reduce the change in resistance effects on temperature.
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,
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.
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??
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.
Thanks for the information.
Hi Emman and Harry,
Sorry to bring up this question again.
When i was going through the datasheet i saw something called Single supply Operation.. !! What does this actually mean?
If its showing 5V single supply operation, does it mean that I can apply ±5V across V+ and V- ?? Or its just 5V and ground across V+ and V-.
Please help me on this as soon as possible.
Thanks in advance,
What datasheet are you referring to? Single supply operation means that one supply pin is connected to ground but it doesn't imply the input voltage range that it is capable to accept. You need to always check and refer to its datasheet regarding the input and output voltage range for you not to violate any conditions for that particular amplifier.
Actually I understand that there are some limitations for the IVR as you said in your previous replies. I wanted to know about single supply operation whether its between ground or not. You have answered that.
Thanks for your help
Before 1975, most op amps ran on +/-15V, and the IVR was +/-12V, or +/-10V or whatever.
So if you ran it on ground and +30V, the IVR was 3V to 27V. National invented the LM324,
which had a PNP input stage, so if you ran it on ground and +5V, the IVR was ground to 3.5V.
So the official definition of single supply is an op amp where the IVR includes the V- rail.
Having said that, all op amps will run on a single supply, but you have to watch out for the IVR.
I was currently doing the testing of the constant current source when i came across a problem which i am not able to figure out why it is happening. Please help me out.
In a buffer circuit the input voltage is measured across a 120k resistor. The opamp used is ADA4522-2.
Case 1:- When a single DMM is used to measure the input voltage, a particular value specific to the current flowing is obtained. When the same DMM is used to measure the output voltage a difference of 20mV from the input voltage is obtained.
Case 2 :- When i use 2 multimeters simultaneously for the measuring the input voltage and the output voltage with their respective negative probes connected to ground, the difference in the multimeter readings has been drastically reduced to less than 0.1mV. In fact when both of them are placed simultaneously, the output voltage(which had a 20mV difference is getting automatically reduced to the same value as the input)
What may be the reason behind this? Why was the buffer giving a difference in input and output at the first case?
For case 1, is the output voltage higher or lower?? (I'm guessing higher)
What is the input impedance of your meter?
Have you tried a different brand of meter?
What is the source impedance of the node you are measuring?
Yes, The output voltage is higher.
The input impedance of the meter is 1MOhms.
I tried with a different brand. In-fact the values both of them showing are almost same.. with some mV difference.
I am probing on the non-inverting input pin and Output pin of AD8639 opamp.
As per the datasheet of AD8639, the input resistance is given as 22.5Tohm and closed loop output impedance is given as 4.2Ohms.
Please help me on this.
Don't know what your schematic is, but if you have a current source driving a 120k resistor,
and you put a 1Meg DVM across the 120k, the current source has a high output impedance
and supplies a fixed current, so some of the current goes through the 1Meg and you get a
lower voltage across the 120k.
Thanks for you reply,
My system is functioning well.. I increased the input impedance of the multimeter and now i am getting the input and output values of the buffer very close to each other.
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