Part Number: INA818
I am currently designing a circuit in which to adapt the measurement of a gauge to an ADC.As I have already commented in other threads regarding the use of the INA818, I have considered a satisfactory output but I have not yet included any RC parallel differential or input filters.
1.- For the differential input filter for this specific device and application, I will use the TI tool, Analog Engineer's Calculator -> "INA Vcm + Dif filter"The help of the tool itself is really good, but I still need to know what frequency range to design.How do I know which one I should consider?And how do I make sure that by putting a resistor in series with the Wheatstone bridge connection I am not going to modify the reading?
2.- In the case of the output filter, a parallel RC is normally recommended looking for the stability of the amplifier.What specific INA818 criteria and data should you take to make it as beneficial as possible?
I have used this documentation as a reference http://www.ti.com/lit/pdf/tidub00 and the great help of this professional forum
what is your signal frequency range? The datasheet of gauge often specifies the bandwidth and settling time.
And the datasheet of ADC usually specifies the component values of necessary charge kickback filter.
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In reply to kai klaas69:
Thank you Kai!
Happy to help. In addition to Kai's questions, can you also provide the gauge and ADC part number?
-Tamara Alani Learn more about IAs here.
In reply to Tamara M Alani:
Hi Tamara and Kai, thank you both very much.
I don't have any defined, I mean the theoretical aspects to be able to apply to any gauge and ADC.
If you wish, we can propose an example, but I can't find the data that Kai asks for. I've been looking at HBM, RS Pro, Omega .... and the gauge's frequency range is not mentioned in the datasheets. The measurements are in DC according to the scheme that I proposed.
For the ADC, I was thinking of using an MSP430 Launchpad or Arduino, for example. But once again, I do not know what aspects are influencing, in this case, for the INA output RC filter.
In reply to SrgiO:
In order to better position ourselves, I add the following screenshots.
This is specifically the configuration at the exit of the INA proposed in the schematic included in the tidub00 documentation from Texas.It's really necessary? Why these values and not others? How to adapt it to each INA and ADC?
In other schematics I have found that the resistance in series with the output is put after the RC parallel. Is it similar to the previous one? Does it change something?
Ri stands for a typical load.
C1 in parallel to Ri is a no-go. 100nF is far too much load capacitance to be directly connected to the output of INA818. If you intend to connect a capacitive load, then do it as shown in the upper scheme with an isolation resistor of R8 = 47...100R or more, if the circuit allows.
By the way, do you know this appnote?
Thank you for sharing that app note, Kai!
Kai is correct. Ri is just the typical load. I would not recommend hanging a large capacitor off the output like that, unless there is an isolation resistor. A large capacitor at the output may lead to significant stability issues. Have you seen our TI Precision Labs training videos on op amps? We have a chapter on stability: https://training.ti.com/ti-precision-labs-op-amps-stability-introduction?context=1139747-1139745-14685-1138805-13848
Hi Tamara and Kai,I have been the time that I have been able to see the interesting information that you have sent me.Once again I thank you for helping me to increase my electronic knowledge. It is very good information!
So any resistive or capacitive load suppose a risk to stability or loss of precision.Is Ri then a load that imposes the ADC?that is, Ri is not a resistance that must be "soldered" at the INA exit, is it? Therefore, is it better not to put any component if it is going to be connected to an ADC?
Some ADCs may require an input filter, here is a circuit example with explanation: http://www.ti.com/lit/an/sbaa282a/sbaa282a.pdf?&ts=1589839598711
I think you can omit the charge kick back filter. Just use a simple isolation resistor, R8 = 100R...220R:
Thanks for sharing, Kai!
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