OPA202: Designing with OPA202IDBVR

Shibin,

1. The topology you are showing is commonly used for driving capacitive loads.  The feedback path includingR242 and R241 should keep the output (after R239) from having a voltage divider effect.  This topology will have the disadvantage of limiting the output swing somewhat.  Normally the OPA202 can swing about 750mV to the supply rails without a load.  For a 10k load the output can swing within 900mV from the rail.  Adding R239 will further limit the output swing by Iout x R239.
2. Most op amps will have an integrated short circuit protection limit integrated into the device. The OPA202 short circuit limit is 35mA.  That means the output can be shorted to any voltage inside the power supply rails of the amplifier and the device will be safe from damage.  However, the short circuit limit protection does not protect against faults beyond the supply rail.  For amplifiers that are connected to the external world can have overstress signals that can damage the device.  This is a reason that Schottky diodes and a series resistor are sometimes used as you show.  However, it is important to put transient voltage suppressors on your supplies.  This is because any external overstress signal that is connected to the op amp output will be routed to the supplies.  The supplies cannot generally absorb transient overstress signals so a TVS diode is added to the power supplies.  In your case, since you are connecting to an external cable I think it is possible that you will see an overstress signal, so your protection circuit is a good idea.  I just wanted to mention the integrated short circuit protection for completeness. 
3. I am not sure which component in your circuit is the PTC fuse.  I suspect it is R239.  Using a PTC fuse in place of R239 would help to protect the output.  When the PTC is triggered, it will take some time to return to its nominal value.  That is ok as you have the output sense path that I mentioned in item 1.  After the PTC is triggered the value of the fuse will be high for some time (even after a reset period).  This will only limit the output swing on your application.
4. I may be missing something but I don't see a major issue with your circuit.  The only change I would recommend is the TVS diodes on the supplies.  The TVS diodes should have a standoff voltage equal to the nominal supply voltage.  You should not see a voltage divider effect.

Let me know if I am missing something.  We can simulate this circuit and the output after R239 will track the input regardless of the load.

Art

Hello AK,
Thank you for your reply.
I am planning to use the PTC after the resistor R239 and the PTC will be the feedback loop of the opamp.
I hope since the PTC is in the feedback loop, there will not be any voltage divider effect.
Please correct me if I am wrong.

The below circuit follows the opamp from the first image attached.
We need both analog voltage (0- 10V) and analog current (4-20mA) out of the same connector.
So we are using a mux (TMUX6219DGKR) to switch between analog voltage and current.


The resistance R235 forms a voltage divider with the load resistor.
When the load resistor is set to 500 Ohms, the analog voltage output was 9.7V corresponding to 5V from DAC.

Can you please comment on whether we can use a current limiting diode like (CMJDH220 TR).

Shibin,

1. Placing a PTC fuse inside an op amp feedback is a reasonable approach.  This document shows this approach to limit the amplifier output current:  slides-protecting_low_voltage_adc_-_improved_solution.pdf on slide 2.  You can also watch this in video form: Protecting low voltage ADC - improved solution .Placing the protection element inside the loop is a good approach to limiting the DC output current of the amplifier.  However, keep in mind that the feedback is a path back to the inverting input, so the overstress signal may be applied to the inverting input.  In your case, it seems that the path to the inverting input has a 7.5k resistor, so this will offer current limiting and some protection.
2. Regarding your overall scheme:  It's really not clear to me how the mux will be incorporated into this scheme.  Are you are planning on connecting the feedback of U39 through the MUX and on the opposite side of R235?  If so, I don't think that is a good idea.  Rather, I would suggest the mux output contains an amplifier configuration similar to what is in the PDF above.
3. If you want further advice on your circuit please draw a complete schematic illustrating all your intended circuitry.  

Best regards,

Art

Hello AK,
Thank you for your reply.
The feedback of the U39 will be from the resistor R239 and the PTC and not from the output of the mux.
The S2 pin of the mux will have analog current output.
The MPU will select whether we need analog voltage (0 - 10V) or analog current output (4 - 20mA).

If we want to use a buffer at the output of the mux, we may need another opamp that can drive a high capacitive load of the cable.
Please correct me if I am wrong.

Looking for your reply.

Vijay,

That makes more sense.  I do recommend that you add a buffer at the output of the mux.  Note that the configuration you show for U39 is intended to drive a capacitive load.  That is the main purpose of that configuration.  Furthermore, that configuration is design to drive a resistive load and avoid the voltage divider effect.  Finally, the series resistor in that configuration (R239) in your case has the secondary effect of limiting the output current (or fault current if the amplifier output is connected to a voltage outside the supply range of the amplifier).  My point is that the configuration used for U39 should be the same configuration as the output buffer.  Furthermore, you may consider simplifying U38, or eliminating it as this amplifier will no-longer be driving capacitance.  If you can tell me the range of capacitance you need to drive, I can help you select the components in that circuit.  Also, you should probably place your PTC fuse in series with a fixed resistor and select the PTC to have a low normal resistance.  In this case, the PTC would act as the protecting mechanism and the fixed resistor would act as the capacitive drive isolation resistance.  You do not want to have to rely upon the PTC to have a specific resistance value as they very a lot.  The PTC resistance will especially change a lot after it is tripped (i.e. there is a long reset period where the resistance value is changing).  

Best regards,

Art

Hello AK,
Thank you for your reply.
If I understand you correctly, I have to modify the opamp to a non-inverting amplifier with a gain of 2V/V and I should add another buffer at the output of the Mux (U38) so as to drive the high capacitive load of the cable.
The output of the mux can be either analog voltage (0-10V) or analog current (4-20mA).
If I make a buffer at the output of the mux, will it affect the current output?

Looking for your reply.

Shibin,

1. One important point.  For the output to drive 4mA to 20mA, you cannot have a buffer.  The buffer will allow you to drive voltage only.
2. The circuit suggestion is very close to your original circuit.  The voltage output op amp can use a PTC fuse, as well as a series resistor to protect and stabilize the circuit for capacitive load.  This circuit will force an accurate output voltage for a wide range of load resistors.  However, you will see some drop across the mux itself for heavy loads.  The mux resistance is fairly low (about 2.1 ohms), so you shouldn't see much error.  Nevertheless, you really don't have any option as the current output cannot go through the buffer.
3. If you want help selecting component values, please let me know what your range of output capacitance is.

Best regards, 

Art