• State
  • Locked Locked
  • Replies 6 replies
  • Subscribers 46 subscribers
  • Views 4645 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

INA128 Saturation Point for 0V to 10V non-amplified circuit supplied with 24 volts

CJB
Prodigy 40 points
Other Parts Discussed in Thread: INA128

I am feeding multiple sensors into a multiplexed ADC.

One sensor has a differential output and is being fed into an INA128 chip and amplified with a gain of 150 (using 333ohms). This amplifies the original value to somewhere less than 2Volts and I'm happy with that behavior.

The other sensor has a single ended output of 0 to 10 volts. I am feeding this sensor through it's own INA128 so that the two sensors will have a common ground. If I don't feed the 10v sensor through an INA128, it's floating ground causes the other sensor's output to mimic the 10v sensor's output (interference).

So far I am successful in routing both sensors through their own INA128s and knocking out the interference from one sensor to the other.

Here's the question... The 0 to 10v sensor saturates it's INA128 so that the output only reads around 8.5 volts when it should be reading 10 volts. The gain is set to zero (no resistor and no amplification) and the supply voltage is around 24 volts. When the supply voltage is only 5, the saturation point is much lower (something like 2 or 3 volts out when it should be 10). What would I have to increase the supply voltage to the INA128 to in order to see a direct 1-to-1 maximum 10 volt value from the sensor and 10 volt out of the INA128 ?

Also, the data sheet seems to say that the max supply voltage is +/- 18 volts, whereas the web site seems to say 36 volts max for supply voltage. (is the range of -18 to +18 being 36volts the reason for the web site saying 36 volts?)

If I get a DC/DC converter with 36volts for the supply of the INA128 would I be able to see the 10 volts from the sensor make it all the way through the INA128? 

 

Thanks in advance,

Chris

  • 0
    TI__Expert 7185 points

    Chris,

    Without seeing your schematic my guess is that at least part of your problem is common mode saturation off the inputs and saturation at the output at low input values.  If you look at the specification table in the INA128 data sheet you'll see the following:  "Common mode voltage range" --> V(+) -2V and V(-) +2V and :"OUTPUT" V(+) - 1.7V and V(-) + 1.7V.  In unity gain to avoid violating the common mode input specification this means that your common mode bias should be at least 2V.  This way, when you apply an input signal the common mode voltage seen at the (-) input of the INA128 will be >= 2V.  Likewise, you can use the same 2V bias and apply it to the reference pin to ensure that the output stays out of the ground rail for low input voltages (i.e. 1V).  You can create this 2V bias either by a buffered divider off the 24V supply and use pull up resistors or you can use the output of the buffer to drive the (-) input directly and let your sensor voltage ride on top of your common mode bias.  How you choose to do this is based on the type of sensor that you have.  Make sure that when you apply the bias to the reference pin that it is a low impedance bias as a straight resistor divider will cause errors in the output diff amp.

    You can also solve this problem by using split supplies, (i.e. +/-12V, +/-15V, +/-18V).  This way, the common mode inputs and the reference voltage to the output will be centered about the supplies.

    With respect to your question about +/-18V vs. 36V--this is saying the same thing.  The 36V refers to the total difference in supply voltage, Vs.

    Matt

  • 0 in reply to Matthew Hann
    Prodigy 40 points

    Thank you.

    Would you be able to recommend a +/-18V supply that I can drive with my 24 to 27 vdc dc power supply?

    I'm finding a lot of +/-15V supplies, but not seeing too many +/-18V supplies.

    Thanks,

    Chris

  • 0 in reply to CJB
    Prodigy 40 points

    One addition, I could tap into my 5vdc dc/dc converters output.

    I've found some 4.5 to 5.5 vdc in, +/-17 vdc output converters.

    I've ordered some samples and will keep you posted on the outcome.

    Do you think +/-17 vdc will provide the solution?

     

    Thanks,

    Chris

  • 0 in reply to CJB
    TI__Expert 7185 points

    Chris,

    +/-17V supplies should work just fine.

    Matt

  • 0 in reply to Matthew Hann
    Prodigy 40 points

    Matt,

    I got the circuit to work and am very satisfied with the results and am now designing this chip into my pcb.

    Our customer has a requirement that all chips on the board be spec'd to an operational high temperature of 125degC.

    On the web sight, I am confused by the specs.

    Can I use this chip up to 125degC or is it limited to an operational high end of "only" 85degC?

    http://focus.ti.com/docs/prod/folders/print/ina128.html#supportandcommunity

    This link is where I see the UA chips listed at 125, but no listing for the P or U chips.

    The data sheet states, "specificaion, +85", Operating, +125, and lists the same for both P/U or UA chips.

    How much does the chip degrade in performance from 85 to 125?

    Please help me understand what I can expect if the chip is operated above 85degC.

    Thank you,

    Chris

  • 0 in reply to CJB
    TI__Expert 7185 points

    Chris,

    Unfortunately, TI does not guarantee device performance outside its specified range, otherwise the specified and operating temperature range would be the same.  With that said, I would expect that the DC specifications for temperature drift to 125C would degrade very little from its 85C operating max.

    Matt