OPA391: Hot swappable not inverting input

Hi Carlo,

what source impedance are we talking about? A very high omic resistor from the input of OPA391 to signal ground would not work?

Why the input must not float? Do you expect a specified output voltage of OPA391 during disconnection?

Can ESD be injected into the input during hot-swapping?

Kai

Hi Carlo,

Are there any specific actions to be considered?

As Kai pointed out, we need more information about the hot swappable input and its characteristics, such as the type of input signals, driving frequency, output impedance, peak voltage/current, transient characteristics etc.. It is likely that the application may require some additional input protection (overvoltage and overcurrent) scheme to protect the OPA391's input during hot swappable interaction. 

The following method is one of the approach, where the input can be float without saturating the op amp.

Please let us know the design requirements. 

Best,

Raymond

Hi E2E community,

Thank you, Carlo, for introducing the community to my problem.

Let me elaborate and give you a better description of our system.

The pH probe can be represented as a voltage source with a very high output impedance, which introduces +414mV/-414mV offset to our 1.74V bias.
Therefore we need an amplifier with very low input bias current (preferably bellow <1pA) and good distribution, such that it doesn't have a large variation between op-amp units.

The output of the op-amp is fed through a LPF to an analog front-end which measures the voltage. The input impedance of the ADC is very low compared to the op-amp's output impedance, so we can neglect it.

As indicated in the schematic, the blue line is an interface (pcb edge connector) between the probe and the op-amp, and they can be disconnected while the circuit is running. There is multiple analog and digital lines being handled by the same interface, but I have simplified it to only include the ones concerning the op-amp.
The R1 resistor is there to provide additional protection from ESD.

I heard that letting the input float can induce unpredictable output oscillations that lead to high current draw and can eventually damage the chip. I do not care about the output while the probe is not connected, but I just want to do all I can to protect the op-amp. 

1. Do you have any recommendation on how to handle the hot-swap without using any additional pull-up/pull-down passives on the op-amp input?

2. In the OPA391 datasheet (p.15 - 7.3.2), it specifies that during continuous use, the differential input voltage should not be greater than 0.5V. Does this limitation also apply in unity gain buffer configuration as shown? The non-inverting input is presented with 1.32V - 2.14V during normal use.

3. Could you recommend any single channel op-amp that could be used in this situation? As mentioned before, Ib should be <1pA with good distribution, and Iq bellow 0.5mA.

Thank you very much for help. I will be looking forward to hearing from you.

Hi Stanley,

1. Do you have any recommendation on how to handle the hot-swap without using any additional pull-up/pull-down passives on the op-amp input?

In pH application, you may hot swap the pH electrode without damaging the op amps. My reasons are the following: pH solution and measurement are passive measurement (other than the typical 512mV biasing reference voltage). The pH electrolyte is ironic and conductive in solution; it should not have ESD or other charges that being built-up during the swapping procedure. 

In addition, LMP7721 has built in ESD protection, which is designed to handle ESD related event outside of a circuit at the op amp's input. 

Yes, the op amp's output will be saturated, but that is not a concern. All pH instruments are behaving this way, which pH reading is displaying Overload (O.L.) or "- - -" symbol or other saturation symbol, when pH probe is not connected to a pH measurement system. 

https://www.ti.com/lit/an/snoa529a/snoa529a.pdf?ts=1630516840245&ref_url=https%253A%252F%252Fwww.google.com%252F

2. In the OPA391 datasheet (p.15 - 7.3.2), it specifies that during continuous use, the differential input voltage should not be greater than 0.5V. Does this limitation also apply in unity gain buffer configuration as shown? The non-inverting input is presented with 1.32V - 2.14V during normal use.

In Section 1.3 of the application note above, there is an amplifier selection guide. If you want to select an op amp different from LMP7721 for sourcing the reference voltage, I can look it this. 

3. Could you recommend any single channel op-amp that could be used in this situation? As mentioned before, Ib should be <1pA with good distribution, and Iq bellow 0.5mA.

You need two op amps for discrete pH measurement. For sensing pH potential, LMP7721 will be one of the best option. For sourcing the pH reference voltage, you may use OPA391, which it meets your design requirements. 

Enclosed is an integrated pH IC solution. It depends how accurate the pH measurements need to be. 

https://www.ti.com/lit/ds/symlink/lmp91200.pdf?ts=1630518248015&ref_url=https%253A%252F%252Fwww.google.com%252F

If you have additional questions, please let me know. 

Best,

Raymond

Hi Stanley,

the 1M resistor at the input of OPAmp not only protects the OPAmp but also the ph probe itself from output shorts into the protection diodes of turned-off OPAmp.

But I don't understand why you cannot power down the OPAmp before removing or replacing the ph probe.

Kai

Hi Kai,

why you cannot power down the OPAmp before removing or replacing the pH probe.

In pH measurement application, users tend to disconnect pH sensor and hot swap with a different one rather than powering down the measurement unit sometimes. The designer is considering the worst case scenario that is occurring in actual practices. 

Raymond 

Hi Raymon,

First of all, thank you for your extensive answer.

The information about potential problems when floating op-amp inputs is described in application note SBOA204A
That is the only reason I am afraid of letting the circuit run with unconnected probe.

I already have the full analog front-end solution for sourcing the bias voltage and measuring the output of the op-amp.
The LMP7721 you proposed looks very nice. However, the supply current is 1.5mA compared to 30uA in OPA391.
Would it make sense to use a MOSFET to turn the LMP7721 on and off to save power while not measuring?

Thank you.

Hi Stanley,

Would it make sense to use a MOSFET to turn the LMP7721 on and off to save power while not measuring?

Certainly, that this may be an option. I speculated that the pH measurement system is likely powered by battery. If it is bench grade pH measurement unit, you will not be worry about Iq of ~1.3mA. 

If you are able to reduce the power supply rail to1.8V, the quiescent power consumption in LMP7721 will be 1.8V * 1.3mA = 2.34 mW. If this is battery powered unit, you should be ok if you are using lithium ion rechargeable battery in 18650 or 26650 type. One of these will power the pH measurement system for a day or more without issues. 

LMP7721 is still high performance op amp for the chemical sensing application. There are others, but I_bias in the part is still exceptional when operating at room temperature. 

If you are  concerned about the power consumption in LMP7721, you may also implement an auto shunt off timer system. When the system is not operating, the timer will turn the system off automatically. 

If you have additional questions, please let me know. 

Best,

Raymond