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Regarding the INA 122 Instrument Amplifier

Ken19261
Prodigy 130 points
Other Parts Discussed in Thread: ADS7861, INA122

I am using the INA 122 for my project in constructing a motor controller (INA 122 connect to ADC (ADS7861) then to FPGA). However, i cant manage to obtain any amplified value.

I designed according to the INA 122 datasheet (Figure 6: Single supply current shunt ) with some modification. Instead of using shunt resistor, i use a voltage divider(R1 = 5100, R2= 120) and the input voltage(after the voltage divider ) i tried from 0.01V to 0.05 V. I used Rg= 2200ohm. Therefore, i suppose to obtain gain of 95.6. However, the output voltage for INA122 seen like saturated as it showed around 2.3V for every different input voltage that i have tried.

I have checked my connection but all pins are connected properly(The pin 4 and 5 are connected to ground while 6 is the output). I hope that someone can help me figure about this as i have no idea for these result anymore.

Thank you in advanced.

  • 0
    TI__Expert 7185 points

    Ken,

    In high gain the INA122 has internal nodes that will saturate when you attempt to common mode to ground; therefore, if you are using this device in a single supply configuration (i.e. +5V) you will need to shift your inputs up to some common DC value (optimally mid supply) to keep the internal amplifiers from saturating.  The figure to which you are referring is implementing a low side sensing technique, if it is possible consider measuring on the high side as this may in itself move the common mode voltage up high enough to keep the differential gain in a linear region of operation on the INA122.  Also, consider that the output needs at least 150mV from ground to keep from saturating and this is accomplished by applying a DC voltage (i.e. a level shift with a low impedance drive) at VREF.  Output saturation into the ground rail is usually an issue with the differential sense voltage is either small or near 0.

     

  • 0 in reply to Matthew Hann
    Prodigy 130 points

    Hi, mark. Thanks for your help. It really help me and now i can process for the next stage already. Well, i use the spice stimulator Ti-7 before i construct the circuit. May i know  Ti has any spice model for ADS7861. I cant found any from Ti website. I am asking this from you since you are this company employee, rght?

  • 0 in reply to Ken19261
    TI__Expert 7185 points

    Ken,

    TI does not have a macromodel for the ADS7861.

    Matt

  • 0 in reply to Matthew Hann
    TI__Expert 5515 points

    And we don't for any of our precision data converters. I've started a new conversation here to discuss what people might want in models for data converters.