OPA4192: Design Inquiry.

Abhishek,

1. You should connect your unused op amp in a buffer configuration with the input grounded.  See  The Unused Op Amp—what to do? 
2. When using an op amp to drive a capacitive load you should use an isolation resistance.  The OPA4192 data sheet has details in table 3 that help you to choose the proper value of RISO.  For a 0.1uF capacitor the table indicates that you could use between 6.2 ohms and 15.8 ohms for phase margins of 45deg to 60deg.  45deg is considered to be the minimum acceptable phase margin, so to have design margin you should choose larger that 6.2 ohms.
3. For your circuit, the isolation resistor and capacitive load are ok.  I am not as confidant in your op amp connections. This will not work as a unity gain voltage configuration (standard non-inverting buffer).   This will also not act as a unity gain inverting amplifier (G = -1V/V).  It could work as a transimpedance amplifier, but I don't think that was your objective.

I hope this helps, Art

Abhishek,

Response for first case:

1. Your questions: The AD3552R eval board uses switches to configure gain, is this ok?  Yes.  This is a pretty common thing to do.  You should take into account any on-resistance of the switch as it can effect gain accuracy.  Mechanical switches have very low on resistance and generally do not impact gain accuracy.  FET switches can have high on resistance and can impact gain accuracy.  Mechanical switches wear out (they have a limited number of times they can be switched before they break).

2. Your questions: The DAC output is a current and a transimpedance amplifier is needed to convert the current to voltage.  Is your configuration ok? Below is an image of a transimpedance amplifier.  Select RF according to your input current and desired output range.  If your input current is +/-1mA, you can choose a 10k feedback to get a +/-10V range (Vout = Iout * Rf).  Choose your Cf according to your bandwidth requirements (f = 1/(2*pi*Rf*Cf).  Do not omit CF as it helps with stability and noise.   Note that the ADI device keeps the RF resistor inside the DAC.  The ADI device has pins that allow connection of the feedback capacitor across RF.  The ADI EVM uses a rotary switch to choose the different feedback capacitors and internal feedback resistors.

3.  Your schematic showing the OPA4192 to the DAC looks good.  The only comment is that placing a capacitor directly on the output of any op amp can cause instability (oscillations).  Small capacitances aren't necessary a problem.  Your schematic shows a 100pF capacitor and that is ok for OPA4192, but I would recommend dropping to 49pF or 10pF.  Your load may have additional capacitance which would add to the 100pF.  Strictly speaking, you can keep the 100pF but I recommend reducing that value.

Response to second case:

1. Your questions: Is the buffer schematic correct. Yes, your configuration is correct.

2.  Your questions: Review the buffer schematic. Your circuit implements a buffer by installing zero ohm resistor in the feedback and do-not-populate for the resistor connected from inverting input to ground.  There is also a zero ohm resistor on the op amp output.  I like this configuration as it gives you the flexibility to change the circuit into a gain circuit if needed.  It also give you the possibility of installing a non-zero output resistor which can help if you have a stability issue.  The only other thing you could add would be DNP feedback capacitors.   I don't think this is needed, but could be added in case you want to have flexibility with this circuit in the future.

I hope this helps.

Best regards, Art

Abhishek,

1. The CF capacitor is in parallel with RF.  CF and RF are generic terms for feedback component.  These components connected from the op amp output to the inverting input.  In your case RF is inside the ADI DAC, and the CF is external (connected on the NICAP and PCAP pins).
2. The isolation resistor should help with capacitive load.  With your isolation resistor you can use either 10pF or 100pF.
3. The supply current is relatively independent of the supply voltage.  See graph below from OPA192:

Best regards, Art

Abhishek,

1. Cascading two buffers will have minimal impact on signal integrity.  Each amplifier will have some noise and distortion, so the total noise and distortion will be slightly worse.
2. Connecting an OPA192 to the input of 14 OPA192 buffers should not be a problem.  Each OPA192 has about 1pF of input capacitance.  The traces will also add input capacitance.  Assuming the total trace capacitance is 10pF, the capacitive load will be 14pF + 10pF.  The OPA192 can drive over 200pF so you should not have any stability problems.  The input impedance of OPA192 is very high so that will not load the amp either.  As a precaution you could add a 10 ohm resistor between the driving amp and the 14 buffers.

Best regards, Art