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AMC1200: second stage amp help

Adam Daluga
Expert 5680 points
Part Number: AMC1200
Other Parts Discussed in Thread: TLV6001, AMC1300

Hello,

We have to worry about common mode voltage range down to 0V at the output.  (The DAS input is +/- 10V, AC coupled, with 50 ohm isolation to ground and 10M between High and Low pseudodifferential input (NI-4497 through a 2144 breakout).  As for the common-mode voltage, it would be ideal to bring that down to 0).

We realize we possibly need a second stage amp(see below) that takes the diff out put of the AMC1200 and converts it to single ended.  This is what we are thinking presently, but want to check with TI to make sure we can still meet our CM requirements. Do you think this would work?

Thanks,

Adam

  • 0
    TI__Mastermind 47595 points

    Hi Adam,

    I don't entirely follow what the question is here - a block diagram or schematic of some sort would be helpful.

    I believe what is being asked is how do I level shift the output of the AMC1200 to be 0V?

    If that is the case then I think this cookbook circuit will be helpful. GND can be used instead of 1.65V to connect to the positive terminal of the amplifier through R1: https://www.ti.com/lit/an/sbaa317/sbaa317.pdf

  • 0 in reply to Alexander Smith
    TI__Expert 5680 points

    Hello Alex,

    This is what I’ve been simulating in TI TINA for the circuit so far. I am going to use a resistor divider where VG1 currently is to scale down the sensor input to be within +/-250mV. VG1 is a 250mV sine wave in the simulation. The input power of the AMC1300 will be isolated from the output power using an REE-0505S isolated 5V-5V DCDC converter. I used the TLV6001 to bring the common mode voltage level from 1.44V to 0V and to convert the signal into a single-ended signal as shown in the TI document sbaa229 (Interfacing a Differential-Output (Isolated) Amp to a Single-Ended Input ADC). This single ended output along with the appropriate ground will be connected to long (200 ft+) wires going to the NI-4497 DAS.

     

    With this usage case in mind:

    1. Is the TLV6001 the appropriate choice to drive the output signal to our DAS?
    2. Are there additional components needed to interface our outputs to the pseudodifferential input of the DAS?
    3. Does the TLV6001 provide enough current for this type of setup? What kind of a load resistor would we use to keep the output centered at 0V
  • 0 in reply to Adam Daluga
    TI__Mastermind 47595 points

    Hi Adam,

    Looks good to me. 

    I don't support the TLV6001 so I'm not confident about it's selection. You may want to post on the general purpose amplifiers forum to get that teams opinion. The TLV6001 has an Open-loop output impedance of 2.3k, so you may need to suggest an amplifier with a higher value. 

    From a common-mode noise perspective, it would be preferable to perform the differential to single-ended conversion after the 200ft+ wires, near the NI-4497 DAS so that any common-mode noise picked up along the 200ft+ wires can be canceled out before the input of the ADC. However, if the GND line for the pseudo-differential input will run along with the single-ended output, this accomplishes the same thing and should be OK. 

    Additional components such as an RC filter or RC charge bucket may be necessary depending on the architecture of the NI-4497 DAS .