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Hello,
I am developing a board to read a resistive sensor arranged in a wheatstone bridge with four active elements.
Output voltage Vo = Ve(dR/R)
Sensitivity S of the bridge is equal to 20 mV/V
The excitation voltage Ve from 0 to 10 V
The bridge impedance is 5 kOhm
Linearity is ±0,1 %FS
I would like to reduce the number of components, it would be nice to use as few Power Supply Rails as possible. But I have to see how to do that though because there are other sensors besides this one and maybe it won't be possible.
So I am looking for a wheatstone bridge readout IC (Bridge Transducer ADC) that can directly acquire the bridge output signal, ideally it requires no interface circuitry.
Performance is also important, I would like to get the most out of this sensor. I'm just starting with development though so I don't really know yet.
Also it doesn't have to be an IC that is too old but I would like to choose one that is recommended for new design and that doesn't have availability problems.
Could you help me with the choice?
Thank you for your support.
Hi Matteo,
If you want to be able to measure a bridge that is excited with 10V, this will complicate your circuit a bit. This is because the no load common-mode voltage of the bridge is 5V with an excitation voltage of 10V. However, most ADCs can support a maximum of 5V on the analog supply, and those with integrated gain amplifiers typically have common-mode limitations that further restrict the input voltage. Therefore, you would need an ADC that support higher voltage inputs, which typically also requires a bipolar supply.
For example, the ADS125H02 can support up to +/-15V input signals, but this requires a +/-18V supply.
A good 5V ADC for bridge measurements is the ADS1235, which has a very low noise PGA integrated into the ADC. However, you would need to level-shift the bridge output voltage into the common-mode range of the ADC / PGA.
We have a document that explains a lot of the fundamental concepts of bridge measurements and how this relates to ADC selection: https://www.ti.com/lit/pdf/sbaa532
Please review my ADC recommendations and this document and let me know if you have any further questions.
-Bryan
Hi Ryan,
thank you for your response.
I was not clear enough. The excitation voltage: 0 - 10 V is the range allowed according to the datasheet for this sensor. I will use a constant voltage and I don‘t want to use 10 V.
I was also thinking that it is a good idea to use the same voltage also as Vref for the ADC.
Considering that S = 20 mV/V do you think 3 V is not enough? otherwise 5 V?
Which parts can you recommend?
Thank you.
Matteo
Hi Matteo,
Thanks for clarifying what you want to do with your system.
If you will limit the bridge excitation voltage to 5V or less, then many ADCs can be used. I mentioned the ADS1235 in my previous post, which is a very high precision, low noise ADC. You can see a typical bridge setup in section 9.2 in the ADS1235 datasheet.
The ADS1260 is a superset part for the ADS1235, with a few more features. Additionally, the ADS1260 has all binary gains between 1 and 128, while the ADS1235 only offers gains of 1,2, 64, and 128. These additional gain values might be important depending on the excitation voltage you choose
I would recommend using the largest excitation voltage possible e.g. 5V, because this will give you the largest output signal from your bridge. As I mentioned previously however, this will impact the largest gain you can use. For example, if you excite a 20mV/V bridge with 5V, the maximum output signal is 100mV. If VREF = 5V, then the maximum gain you can use must be less than 5V / 100mV = 50. Therefore you would need to choose a gain of 32.
Also note that the two ADCs I have mentioned here only support AVDD = 5V. Another option if you want AVDD = DVDD = 3V for example would be ADS124S06. This device has slightly higher noise compared to the ADS1235 / ADS1260, though this depends on the ADC output data rate.
-Bryan
Hi Brian,
thank you for your support.
While I'm looking into the products you recommended, I wanted to ask you what do you think about an IC like PGA302 or in general choosing a programmable gain amplifier to use with a bridge?
Thank you.
Matteo
Hi Matteo,
Since all of the ADCs I mentioned have an internal PGA, I am not sure you would need an external amplifier unless you want to use an excitation voltage >5V. However, as mentioned earlier, this will generally require higher voltage supplies
The PGA302 could be used in this application, assuming 16-bit performance is sufficient. The main feature of the PGA302 is that it offers a 3rd order temperature and linearity compensation algorithm, if this is an attractive feature for your application.
-Bryan
Hi Bryan,
I didn't mean to use a PGA in combination with one of the ADCs you suggested. I was asking if it could also be a solution instead of one of your ADCs to choose a PGA with an internal ADC.
PGA302 was just an example, I have seen that there are so many of these PGAs.
What do you suggest?
Thank you.
Matteo
Hi Matteo,
Since I don't know all of the parameters of your design e.g. cost, size constraints, power consumption, accuracy, etc., I cannot really tell you which device to select. All I can give you is guidance on the pros and cons of each solution, which is included in my previous posts and restated here:
-Bryan
Hi Bryan,
Thank you for your reply.
At this stage it is not important but in the future certainly, so yes I am looking for: small footprint, inexpensive, evaluation board available, consumption is secondary.
So if I don't use the ADC in the microcontroller then you recommend one of the ADCs mentioned above and not one of the PGA30x (the only advantage would be the compensation which I can do with the microcontroller though) right?
Hi Matteo,
The PGA30x devices offer an all-in-one solution that includes multiple ADCs (one for bridge measurement, one for temperature compensation) and an MCU to perform the linearization and compensation. Yes, you can perform all of these functions discretely, so it's really up to you to determine how much control you want to have over the overall signal chain. The PGA30x is therefore the most integrated solution, while the discrete amplifier + MCU ADC is the least integrated, with the precision ADC somewhere in between.
-Bryan