AD8397 : Clarification regarding output swing range

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

• We are using AD2S1210  in our design. For the excitation Coil output of this IC, You recommend using a filter circuit
• This was also put forth by you in circuit note CN0276 
  • In this, AD8692 is used in the first stage and AD8397 is used in the Second Stage
  • To get a TYP VPP of 4.60 V output from AD8397(This is the voltage level we are interested in), the AD8397 is configured at 1.28 gain
  • The same circuit is simulated and we observed clipping at ~1.6 V
  • The screenshots of the simulations are included below. Please have it for your reference.

Query:

• Can you please let us know if the observation made in the simulation is correct
  
  • if yes please confirm whether it is supposed to be clipped off at 1.6 V in the actual circuit and still have no effect on the performance of AD2S1210
• Also, in an earlier question, I asked :
  • Can VCC and VEE be provided with different voltages?(Example :VCC=12V and VEE = -5V).
  • If Yes, would this configuration have any long-term reliability issues

Thanks,

Sameep Pai

Hi,

Champ7374 said:

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.

Champ7374 said:

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.

Champ7374 said:

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. 

Champ7374 said:

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

Hi,

Champ7374 said:

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.

Champ7374 said:

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.

Champ7374 said:

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. 

Champ7374 said:

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