Hi, we are now designing an OVP circuit for ADA4807 using littlefuse INC's pulseguard TVS Diodes, and then we found ADA4177 today!
But, the signal we are going to sample has 2.5MHz bandwidth, VPP=5V, unit gain and with low temperature drift, which requires: GBP > 40MHz and slew rate > 30. ADA4177 has only 3MHz GBP and much smaller slew rate. Is there any Op Amp (to be released) with integrated OVP feature that meets our needs ?
Thanks in advance.
Thanks tony and harry for your replies.
The frequency of ESU is 300KHz.
Our device is placed on the head during surgery, which is a little bit far away from defibrillation or ESU. However, ESU affects our device more than defibrillation. Defibrillation emits transient high currents, but these currents mainly flow between the two defibrillation pads. ESU will last around 30s to 2min. During ESU, the op-amp will saturate (Over-Voltage).
We are afraid of using TVS diodes because:
1, The capacitance. In our device, we inject small but safe current into the head (1mA maximum, 10k to 1MHz). There are 16 or 32 electrodes placed at the head, so each TVS provides additional capacitance to ground, which may degrade the accuracy of the measurement of boundary voltages, especially at ~1MHz frequency.
2, The article in
Op Amp Input Overvoltage Protection | Analog Devices
shows that using ADA4177 may provide better performance, especially when the operating (Room) temperature varied from 20 degree Celsius to 37 degree.
A 1k Ohm resistor was connected between VIN+ and the input, and a feedback resistor of 1K Ohm was placed between OUT and VIN-. The op-amp is not only being used as a buffer, but also acts as a shield driver, which is shown in the following figure.
1. Will the current limit resistor worse the output voltage noise? We will left TVS diodes as DNP, and test the results using only the OpAmp and the current limiting resistors.
2. We never find ADA4625 in the list of op-amp (may be it is just being released). The specifications of ADA4625 is impressive (I think it is even better than ADA4627). It can also drive high capacitive load (1000pF). However, it is more expensive (which surely need to be protected )