AD8397
Production
The AD8397 comprises two voltage feedback operational amplifiers capable of driving heavy loads with excellent linearity. The common-emitter, rail-to-rail...
Datasheet
AD8397 on Analog.com
AD2S1210
Production
The AD2S1210 is a complete 10-bit to 16-bit resolution tracking resolver-to-digital converter, integrating an on-board programmable sinusoidal oscillator...
Datasheet
AD2S1210 on Analog.com
AD8692
Production
The AD8691, AD8692, and AD8694 are low cost, single, dual, and quad rail-to-rail output, single-supply amplifiers featuring low offset and input voltages...
Datasheet
AD8692 on Analog.com
Hi Analog, I hope you are having a pleasant day !!
Description of the issue:
The reason I am here is to get some clarification regarding the output swing range of AD8397 full-swing Opamp. I have simulated a simple Voltage follower circuit in LTspice to check the maximum and minimum Voltage at the output. Below mentioned are the configuration and observations made.
Query:
Thanks,
Sameep Pai K S
Hi,
I think it can be due to the input stage. Here is a sentence from the datasheet: "When operating the AD8397 in a unity-gain configuration, the output does not swing to the rails and is constrained by the H-bridge input." and another one: "To avoid overdriving the input and to realize the full swing afforded by the rail-to-rail output stage, use the amplifier in a gain of two or greater. "
What is strange: the input common mode range is not specified in the datasheet. Anyway, I think one should follow the recpmmendation: use gain of at least two if you want to get the rated output swing.
Zoltan
Hi,
I think it can be due to the input stage. Here is a sentence from the datasheet: "When operating the AD8397 in a unity-gain configuration, the output does not swing to the rails and is constrained by the H-bridge input." and another one: "To avoid overdriving the input and to realize the full swing afforded by the rail-to-rail output stage, use the amplifier in a gain of two or greater. "
What is strange: the input common mode range is not specified in the datasheet. Anyway, I think one should follow the recpmmendation: use gain of at least two if you want to get the rated output swing.
Zoltan
Hi Zoltan,
Your swift response is highly appreciated.
Thanks for pointing out that in order to get the rated rail-to-rail swing the opamp must be operated with a gain of at least 2
But this Question initially aroused within us when we faced an Issue while simulating the Filter circuit for AD2S1210.To keep the story short I just simulated a voltage follower and raised the query. Below I have mentioned the Query and what we are looking for in detail
Query:
Thanks,
Sameep Pai
Hi,
Can you please let us know if the observation made in the simulation is correct
I can only guess if it is correct, since the input common mode voltage range specification is seriously missing in the datasheet. So ask an ADI employee, PaulCA to update the datasheet or at least provide this information.
Can VCC and VEE be provided with different voltages?(Example :VCC=12V and VEE = -5V).
The opamp do not have a ground terminal, therefore only VCC-VEE matters. Of course, the signals are ground referenced, therefore the input and output voltages are limited by VCC and VEE. The clipping problem will probably disappear, if you apply negatíve VEE: Give it a try in the simulation.
would this configuration have any long-term reliability issue
VCC-VEE is well within the absolute maximum ratings, so it is safe to operate the opamp with these supplies. Another very strange fact that the input voltage range is not mentioned in the absolute maximum ratings.
no effect on the performance of AD2S1210
I don't really understand what do you mean. What kind of effect could it have?
Just a question: what is the minimum and maximum voltage at the AD8692 output? What should be the corresponding minimum and maximum at the output of the AD8397?
Zoltan
Hello Champ7374,
I have run some tests in the lab using your setup and here are the results:
Dual Supply: Vs = +/- 5 V
Input = 1kHz, 10Vpp
Output:
Single Supply: Vs = + 5 V
Input = 1kHz, 5Vpp, 2.5 V DC offset
Output:
Based on the results above, at gain of 1, the part is still able to provide an output within 0.5 V of Supply Rails. The statement "When operating the AD8397 in a unity-gain configuration, the output does not swing to the rails and is constrained by the H-bridge input", I think this is the reason why there is 0.5 V output head room/foot room from the supply stated on the data sheet. At gain of 1, the output should be still within 0.5 V of the Supply rails as can be seen on the results above.
I do agree with ZoltanDTI that in order "To avoid overdriving the input and to realize the full swing afforded by the rail-to-rail output stage, use the amplifier in a gain of two or greater. "
At gain of 2, the part should be able to provide a rail-to-rail output. Please see the simulation below:
In conclusion, there seems to be a problem with the spice model when configured in Unity Gain (Gain = 1). I will be reporting this issue to the LTSpice Service Desk in order for it to get fixed.
Regards,
Paul
Hello,
Just getting back on this. There is an existing FAQ which contains information about the Input Voltage Range of AD8397. Please see this link: AD8397 input voltage ranges datasheet Rev A - Documents - Amplifiers - EngineerZone (analog.com)
I hope this helps.
Regards,
Paul