INA326 reference voltage

Discussion in 'General Electronics Chat' started by Rokeeeez, Mar 4, 2018.

  1. Rokeeeez

    Thread Starter New Member

    Feb 8, 2018
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    Hello, I want to ask, it is necessary for INA326 instrumentation amplifier connect reference voltage to PIN5? I try to design this circuit on breadboard and i didn't know do i have to connect reference voltage to PIN5? If yes, what voltage? 2.5V or how much? And if I didnt connect anything to PIN5, will it be smaller gain, because I think ref voltage is substracted to output voltage of amplifier? I attach datasheet, circuit on 9 page (single supply).

    http://www.ti.com/lit/ds/symlink/ina326.pdf
     
  2. MrChips

    Moderator

    Oct 2, 2009
    17,381
    5,367
    Pin-5 of INA326 is not a reference input. This is a load path for the current mirrors.

    According to the data sheet, pin-5 must be connected via a resistor R2 to GND.
    The gain is shown in equation (1) as

    G = 2 x R2 / R1
     
  3. ebp

    Well-Known Member

    Feb 8, 2018
    2,276
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    There may be confusion here because there are many integrated instrumentation amps that do have a "reference" input for the output stage. In those amps the output is referred to that signal - it is essentially an offset input for the output stage. Typically it is connected to circuit common but it can be driven with a low-impedance source/sink, such as a fixed voltage reference or the output of another amplifier. Using it can avoid the need for another amplifier stage for level shifting.

    I don't recall ever seeing another IA with a configuration like the INA326. For many applications I would regard its peculiarity as a nuisance, but there are interesting things that it can do conveniently that others can't.

    Figures 10 and 11 in the data sheet are badly and misleadingly drawn (i.e. wrong!). The ZVN4525 does not a substrate pin. What looks like connections to the substrate are actually supposed to be to the gate.
     
  4. Rokeeeez

    Thread Starter New Member

    Feb 8, 2018
    19
    0
    I use a circuit from INA326 datasheet 9 page, except I don't use filtering at output of amplifier. R1=4,7k, R2=100k, so gain could be aproximetely 40-50. On the pin 5, I connect 100k resistor and 0,1uF condensator. Problem is that, when I apply pressure to my transducer voltage between data signals is 30mV. So, for my gain, output voltage could be about 1,2-1,4V, but when i check voltage with multimeter I see only 160mV, and when I applied more pressure to my transducer, voltage in the output of amplifier didnt change, still 160mV. Both, transducer and amplifier powered by Arduino, and I check voltage between amplifier output pin and common ground in the breadboard. What can be problem? I attach INA326 datasheet and my breadboard connected circuit, maybe there are some connection errors.
     
  5. ebp

    Well-Known Member

    Feb 8, 2018
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    Can you post your schematic? A photo of it drawn by hand on paper is OK as long as it is the right way up and we can read it. What voltages do you measure at the bridge output relative to circuit common?

    Are you sure the bridge is connected the "right way up?" Remember that when the amp is powered with a single supply the voltage on the non-inverting input must go more positive than the voltage on the inverting input. Try reversing the connections between the bridge and the amplifier.
     
  6. Rokeeeez

    Thread Starter New Member

    Feb 8, 2018
    19
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    I attach the schematics. Also, I try to reverse inputes from bridge to amplifier when there is no presure cause small change in the output of amplifier - 280mV then, but when applied more presure there are no any changes.

    R1=4,7k, R2=100k, C1=0,1uF

    upload_2018-3-4_20-14-26_Rokeeeez.jpg


    Mods Note:
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    Last edited by a moderator: Mar 4, 2018
  7. ebp

    Well-Known Member

    Feb 8, 2018
    2,276
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    I need to see the schematic for the whole circuit including the sensor bridge. I suspect there is either a connection to the wrong place somewhere or there is a connection missing.

    I think you may be missing a common ("ground") connection between your amplifier board and the arduino board.

    Look carefully at your breadboard. Measure voltages on the pins of the amplifier IC (or the adapter board). It is easy to get a wire in the wrong hole on those breadboards. With DIP ICs sometimes a pin gets bent underneath the body of the part. I looks OK but it doesn't go into the hole. If you probe in the adjacent hole you may see your signal. If you probe on the pin you won't see it. You do have to be very careful when probing pins so you don't accidentally cause a short circuit if your probe slips. If I use a meter for that sort of think I like small probes with very sharp tips.

    In general, when you are troubleshooting, always try to measure a signal as far away from where it originates as possible. For example, if you measure a bridge output signal right at the bridge it may look OK, but if you measure as close to the amplifier as you can get you may find the signal looks wrong, which usually means a connection problem with a breadboard. The idea is to get as much information as you can from each measurement you make. If you find the signal is good in one place and bad in another, then you start making more measurements working from the ends to isolate where the problem is.
     
  8. Rokeeeez

    Thread Starter New Member

    Feb 8, 2018
    19
    0
    I dont know exactly bridge of my transducer, because bridge is in construction of transducer, i have only 4 wires (Excitation + - and Signal + -). There is datasheet of my transducer: http://stevenengineering.com/Tech_Support/PDFs/31DTMAIN.pdf (First, model AB/HP).

    What do you mean common ground between amplifier board and Arduino board? One wire goes from Arduino board, to blue (ground) line of breadboard and then to this line I connect 4 pin of amplifier (V-), also R2-C2.

    Also, I connect transducer Excitation+ to 5V pin of Arduino, and Excitation- to GND pin of Arduino.
     
  9. ebp

    Well-Known Member

    Feb 8, 2018
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    The circuit seems like it should work. I come back to suspecting a connection problem.

    I suggest you pick a single point for "ground" for your meter and probe the inputs and output of the amp, the supply and ground pins of the amp, the ground-side leads of any passive components and both sides of the excitation for the bridge, keeping in mind what I said previously about how to probe signals. Don't assume that because you think something is connected to ground that it is - check it with your meter probe actually on the part pin, not in a hole in the breadboard.

    Very carefully examine your TSOP adapter board and the amplifier pins to make sure the connections are good and there are no shorts.
     
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