OPA830: Circuit design ideas and analysis.

Aaditya Sharma

Part Number: OPA830
Other Parts Discussed in Thread: OPA381, TINA-TI, OPA392

Hello community,

I have designed a strain gauge sensor using OPA381 in a bridge configuration in TINA-TI (attached herewith), taking reference from the application circuit of MAX4239.

When I simulated the schematic, I found that the circuit is working correctly and very precisely for the minute change in resistance of R2, but it does not meet my requirement for the following reasons:

1. My sensor has a very high gauge factor (typically between 5 and 10). Since I need a linear change in voltage with a change in resistance (for example, if the base resistance is 100 ohms, I am expecting a linear voltage change when it reaches to 500 Ohms or so). But typically in the above circuit, in bridge configuration, the voltage saturates within a small range of resistance change (around 110 in above example). Any explanations and ideas to tune these values or modifications!

2. I am wondering why the maximum voltage output is only 1.65 V and it saturates for higher values of resistance.

Thank you for your kind consideration.

Best regards,

OPA831_BridgeRectifier.TSC

over 1 year ago

0 Chris Featherstone over 1 year ago

TI__Mastermind 23040 points

Hey Aaditya,

We are currently looking into your question and will need up to 2 business days to respond.

Best Regards,

Chris Featherstone

0 Marek Lis over 1 year ago

TI__Guru 63970 points

Aaditya,

The issue with your circuit is that it violates the maximum input common-mode voltage range of Vcm<(V+)-1.8V, which on 3.3V supply means that Vcm must be at 1.5V or less.

Lowering R5 to 500ohm lowers Vcm to 1.5V but in order to keep it constant it is R3 that must be changing and not R2 - see below.

This scheme works fine for ideal op amp but OPA381 has linear output limitation defined in AOL test to be (V+)-0.6V for RLoad (R6) of 10k. With R6 of 100ohm output becomes non-linear for R3 above 380ohm - see below.

One way to correct the problem is to lower the R6 to 75 ohm - see below.

Below I have attached for your convenience edited Tina-TI schematic.

OPA831_BridgeRectifier edited by Marek.TSC

0 Aaditya Sharma over 1 year ago in reply to Marek Lis

Prodigy 130 points

Dear Marek Lis,

As you outlined, I also obtained better results by tuning R5 and R6 values. However, I am still curious if we can tune the sensing range between 0 to 3.3V. I am open to changing the op-amps with the same footprint but would prefer to use a similar bridge configuration.
Best regards,

0 Marek Lis over 1 year ago in reply to Aaditya Sharma

TI__Guru 63970 points

With ideal op amp you could make R5 and R6 equal to 210ohms resulting in the output sensing range from 0 to 3.3V - see below.

However, since no op amp (including OPA381) can have output swing all the way to its rail under sinking or sourcing conditions, setting R5 to 260ohm and R6 115ohm results in the optimal output voltage swing from around 0.8V to 2.5V - see below.

Having said that, you could use another op amp like OPA392 to increase the output range within 240mV from either rail - see below.

OPA392_BridgeRectifier edited by Marek.TSC

0 Aaditya Sharma over 1 year ago in reply to Marek Lis

Prodigy 130 points

Dear Marek Lis,

Thanks a lot for your kind support and effort.
I am curious to know if any alternative bridge configurations could give a linear voltage output corresponding to the resistive input. This time we can modify the whole circuit that we designed above but I want to stick with the single supply op-amp.

Best regards,

+1 Marek Lis over 1 year ago in reply to Aaditya Sharma

TI__Guru 63970 points

Bridge configuration results in linear output only for small variation of its resistance. For 5x bridge resistor variation (from 100ohm to 500ohm) one way to keep the output a linear function would be to use just two of the bridge resistors with R4 variable as shown below.