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AMC1200-Q1: AMC1200-Q1

robin gangopadhya18
Mastermind 6086 points
Part Number: AMC1200-Q1
Other Parts Discussed in Thread: AMC1200

Tool/software:

Hello:

Pl take a look at the attached schematic.

A signal of 100mV, 1kHz is applied to the inputs of AMC1200-Q1 through  balanced RC filters.

U2 amp is biased on the input side by 5V. The output is biased by 5v with an isolated return from the input bias circuit.

One would expect outputs at pin 6 & pin 7 with common mode DC values of 2.5V.

But we measure 3.8 V on pin 6, 1.2V on pin 7.

And absolutely no signal amplification at all.gmr10d_test_ckt_ti12052024.pdf

What am I doing wrong?

Is it possible that I got the circuit all wrong?

The real application of this circuit is to measure the voltage across Vac  nodes when it is replaced by a 1Ohm resistor. U1 is a power converter using UCC2811 controller.

We are measuring, in effect, the input current through the U1 converter, but it must be isolated from the instrumentation side, hence the use of an isolated amplifier.

Any review and comment on this circuit will be highly appreciated.

We just bought a whole lot of this amp from Mouser!

-r

  • 0
    TI__Expert 4260 points

    Hi Robin,

    robin gangopadhya18 said:
    But we measure 3.8 V on pin 6, 1.2V on pin 7.

    Is this a DC measurement?  The common mode in this case is 2.5V.  Vcm = (VOUTP + VOUTN)/2 = (3.8+1.2)/2 = 2.5.

    Do you have any waveform captures of the input and output of the device?

    Thanks.

  • 0 in reply to Saleh Ahmad
    Mastermind 6086 points

    Hello Saleh:

    The outputs I measured with a oscillo probe are DC as can be discerned clearly from the values.

    (3.8+1.2)/(2) =2.5 has no meaning!

    Each output has to be 5/2= 2.5v DC.

    Allow me to state that this set up does not need any "waveform" to be presented! Because what you see are  DC values: 3.8V DC on pin 6  and 1.2V DC on pin 7. & there was no "amplified" signal at all.

    Is it possible that the ICs are a "bad" batch?

    I am getting an Eval kit. We will find out which ICs are "bad" by putting our parts in the Eval kit.

    r

    -r

  • 0 in reply to robin gangopadhya18
    TI__Expert 4260 points

    Hi Robin,

    robin gangopadhya18 said:
    A signal of 100mV, 1kHz is applied to the inputs of AMC1200-Q1 through  balanced RC filters.

    Are you getting the 3.8V/1.2V DC output when the input is the 100mV AC signal?

    robin gangopadhya18 said:

    (3.8+1.2)/(2) =2.5 has no meaning!

    Each output has to be 5/2= 2.5v DC.

    The common mode voltage is defined as  (VOUTP + VOUTN)/2.  Across the input voltage range the common mode should stay at 2.5V, so  (VOUTP + VOUTN)/2 should always equal 2.5V.  The input/output relationship should follow the below plot.

    Based on the 3.8V/1.2V measurements you're getting it seems like the input is at or past the clipping voltage.  Have you verified what the voltage at the inputs of the device is?

    Thanks.

  • 0 in reply to Saleh Ahmad
    Mastermind 6086 points

    Hello Saleh:

    If you look at the schematic, input pins are at 0 although "unbalanced".

    I did measure  53mV DC across +/- inputs while I have nothing from my side.

    But this cannot cause output DC CM to shift that much.

    But I will redo tests again & see if I missed 400mV diff voltage at the inputs.

    r

  • 0 in reply to Saleh Ahmad
    Mastermind 6086 points

    Hello Saleh:
    No, we have 0 volts applied at the inputs.

  • 0 in reply to robin gangopadhya18
    TI__Expert 4260 points

    Hi Robin,

    Can you double check the high side supply at the device and then short INP,INN, and GND1 at the device pins, and check if the output still shows a differential voltage.

    Thanks.

  • 0 in reply to Saleh Ahmad
    Mastermind 6086 points

    l as soon as I get the Eval kit & go to the lab.

    thnx much

    r

  • 0 in reply to robin gangopadhya18
    Mastermind 6086 points

    Hello Saleh:

    So I am in the lab with AMC1200B EVM on hand.

    I applied 5VDC across pin8 and pin5.

    Sure enough, pin 6 is 2.5VDC, pin 7 is 2.5vDC. No connection on the input side.

    I do the same on a random Test PCB assy , I measure pin 6 at 1.2VDC,  and pin 7 at 3.8VDC, with nothing on input side .

    There is definitely something wrong with parts we got from Mouser.

    I got 5 samples from TI -SOIC pkg-  will test them now.

    Let you know asap.

    -r

  • 0 in reply to Saleh Ahmad
    Mastermind 6086 points

    Hello Saleh:

    It turns out that the 5 samples we got from TI a yr or more ago are defective!

    The new ones we bought from Mouser do work exactly as the d/s says.

    We have tested from DC to our freq of interest.

    I need to double check the gain, but it is near 5 not 8 but I must test in DC with multimeter...

    If this is going to be 5 for all the AMC1200, no problem.

    will let you know what we find.

    r