Fast overdrive recovery time of OPA699

Hello Luca,

Please see below for some information on the behavior of the OPA699 that you are seeing.

When the OPA699 saturates and the op amp goes open loop, the + and - inputs are no longer at the same potential (no longer virtually shorted). For as long as the input has a large differential voltage (Vid), the output will remain saturated. The output only begins its recovery when this input differential voltage has been reduced such that the ideal output (Vout = Vid*Aol, where Aol = open loop gain) is within the op amp’s linear output range. For a regular op amp without a limiting function, the recovery time can be very long (ns to us), but for the OPA699 which has a limiting function at the output, this recovery time is limited to 1ns. This is the case when the OPA699 is configured in a regular non-inverting or inverting gain configuration. When the input signal changes and the input differential voltage reduces, the output quickly recovers from saturation and resumes linear operation.

However, this is not the case in a transimpedance circuit. Even when the input current signal has been reduced into what would be the ideal linear region, the input differential voltage persists until the voltage on the capacitances at the inverting node has discharged sufficiently through Rf. Simulations will show this. See the attached “OPA699 TIA” Tina simulation.

One way to take advantage of the limiting function of the OPA699 (if this is to go to an ADC, for example) is to cascade a TIA stage using a very low noise op amp such as the OPA847 followed by a regular gain stage implemented using the OPA699. The gain of the transimpedance stage should be limited to ensure that no input will cause its output to saturate. The OPA699 voltage gain stage can make up the rest of the gain without worry of saturating due to unwanted inputs. See the attached “OPA699 TIA composite” Tina simulation. The initial OPA847 stage provides a transimpedance gain of 7.1 kΩ and the subsequent OPA699 stage provides a gain of 6V/V. The total gain is 42.6 kΩ.

For example, say the desired input signal is +/-20uA but there is the potential for the input to spuriously go as high as +/-100uA. With a TIA gain of 7.1kΩ the +/-20uA will be amplified to +/-140mV and the +/-100uA will be amplified to +/-710mV. This is within the linear output range of the OPA847. Since the original +/-20uA is the typical, desired signal, the gain of the OPA699 is set to 6V/V to amplify the signal further to ~1.7Vpp (for an ADC). The undesired +/-100uA signal (+/-710mV at OPA847 output) causes the OPA699 output to saturate, but the OPA699 quickly recovers back into linear operation.

Another option is to use either back-to-back diodes on the input pair, or a single diode to keep the input within the Vlim range; however this would add capacitance on the input. This may work, but the above may prove to be the better solution.

I hope this helps answer your question, please let me know if you have any further concerns.

Regards,

Ranji C. Bhola

High-Speed Amplifiers

3247.OPA699 TIA.TSC

3782.OPA699 TIA composite.TSC

Hello Luca,

The noise and jitter is definitely prevalent in your above mentioned solution, and it is stemming from the LMH6642, the jitter may be coming from the slew rate limitation in the LMH6624.

I would recommend using an OPA656 instead of the LMH6642 as it has a high TIA gain, very low input bias current, and FET inputs to have very low noise; however there will not be rail-to-rail operation in that device. Then you may try an OPA699 or OPA847 to replace the LMH6642.

The OPA699 would help the jitter if the cause is solely the slew rate limitation.

The OPA847 would help the jitter if the cause is the Bandwidth limitation.

As for the solution I mentioned to you in the previous post with the OPA699 and OPA847 the testing would be able to verify if this solution is a better option vs your solution above.

For either case I would recommend powering the devices in the manner depicted in the attached image as this will help with PSRR & Oscillations. (Example below uses OPA656 and OPA699 or OPA847)

Let me know if you have any further questions and how this turns out for you.

Regards,

Ranji C. Bhola

High-Speed Amplifiers