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hi,
Need your help to understand and fix an unexpected results while using INA819 as amplifier with gain = 6.9
The op amp's power is single 5 volt dc and Gnd
It looks very simple, but does not work
c81, c78 , c79 are not assembled.
Output is going to TI's ADS7828E Analog to Digital converter
The purpose of the design is to measure 4 to 24 mA dc current
The ref pin voltage is 2.5 volt driven by OPA191
I apply +/- 0.4 volt at input and expect to see + 5 volt or 0 volt at output (depend on polarity)
The amplifier starts to sense the input voltage only above +/- 1.1 volt ( measured between legs 1,4)
Only when input voltage is about +/- 2 volt DC the output goes to 5/0 volts in correspond
I don't know how to solve it
please your advice
regards
Tsfania
Tsfania
Hi Tsfania,
There are several issues with the circuit.
1. If the differential input is ±0.4V with Vcm near ground, then you have to use bipolar supply voltage in configuration. If you are using 5Vdc as a single supply, then the input Vcm has to be configured above GND; the Vcm is optimized at the mid supply of 5Vdc or Vcm = 2.5Vdc. Please specify your Vcm mode voltage in the circuit.
Enclosed is the analog engineer's calculator, which it is used to check the linearity of differential input for given Vcm vs. Vout conditions.
https://www.ti.com/tool/ANALOG-ENGINEER-CALC
2. Rather than using ferrite beads to attenuate higher frequency (>10MHz), we may implement differential and common mode filters as shown at the input. The instrumentation amplifier(IA)'s input needs to be balanced and impedance should be matched. Otherwise, common mode noise may turn into differential signals, which are not part of the design interest. Ferrite beads does not precise impedance and its tolerance can vary up to +/-25%.
The differential capacitor should use NP0/CG0 type or you may use X2Y capacitor, which you should be able to purchase from mouser.com. However, you may place a ferrite bead at the power supply rail for the INA819 IA.
Regarding to the differential and common mode LPF filter calculation, please see the attached image below. You may find the reference in section 3.2.1 in the attached link below.
https://www.ti.com/lit/ug/sbou115c/sbou115c.pdf?ts=1646145708887
https://www.mouser.com/datasheet/2/611/x2y_filter_and_decoupling_capacitors-1771721.pdf
3. The IAs or Amplifiers do not like to drive capacitive load directly, which it will introduce pole within the feedback loop and create instability. Please remove the C81. You may place a low pass filter after INA819.
Enclosed is a schematic that is similar to what you have. Since I do not have all the design requirements, please consider this simulation as an example.
BTW, ADS7828E is SAR ADC, please check with our ADC supporting team. You have to implement Anti-aliasing + charge bucket filters in order to optimize the input filters. If system noise is a concern, you may require to perform noise analysis or calculation to optimize the setup.
If you have additional questions, please let us know.
Best,
Raymond
Hi Tsfania,
The following Vcm vs. Vout is fairly close to your current IA setup. The circled orange figures are shown in min. and max. differential input should not go beyond ±0.35V on top of 2.5V. Your low end of input signal should be approx. 2.5 - 0.35 = 2.15V; and high end of input signal should be approx. 2.35V. So the input between 2.15V to 2.35V is your instrumentation amplifier's range, which Vout will keep a linear relationship with the input.
You mentioned that "the main problem is that the ina819 output do not respond to voltage at input until +/- 1.1 volt dc"
If I understood your statement correctly, your input is outside of the required input range of INA819 per the configuration, which it will not be linear. If INA819 responds to the input, it does not mean that Vout will be in a linear relationship.
If you applied +/-1.1V at the input of INA819, that means that your Vcm is not centered at 2.5Vdc. Please check with the Analog Engineer Calculator above and below Vin- and Vin+ in Max. and Min. requirements.
Based on your latest schematic, I do not see that the Vin+ and Vin- nodes have the common mode Vcm = 2.5Vdc. Can you show me what are Vcm at P1 and N1 at J5 node? It is some kind of sensor or wheatstone bridge.
Best,
Raymond
Hi Tsfania,
Is it necessary to add protection at the A2D inputs against negative voltage that may occur ?
I am fairly sure that you will need to add anti-aliasing filter and kickback bucket filter in front of SAR ADC. You may use single supply rail op amp for the filter design and will clamp the output to GND, if INA819 may source negative output by accident and/or wrong sensor connections at input. Please consult these design requirements with ADC supporting team.
Best,
Raymond
