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Original question:

AMC1350: Spice model

AMC1350: High noise observed while testing the voltage sensing circuit using AMC1350 in power board

Aswathy P N
Prodigy 30 points

Part Number: AMC1350
Other Parts Discussed in Thread: TLV9051

Given below is the schematics and simulation diagram of the battery voltage sensing circuit used in the chopper board. The marked section in the simulation diagram is the input side of the AMC1350 IC. The voltage divider is connected like this inorder to operate the battery reversal operation also. That is need to sense the voltage -900V to +900V. Can you verify the below circuit is correct or not ? We observed noise at the input of AMC1350 IC(shown in the below figure). Is it because of the way we connected the resistor network ? Please help to rectify the issues.  image.png BATTERY_IMAGE.png

  

In waveform  , Red: Input of AMC 1350 IC​

                        Blue: Output of AMC 1350IC

 

 

  • 0
    TI__Expert 4281 points

    Hello,

    I believe the problem is in the differential amplifier circuit rather than in the AMC1350. Let me share my thoughts combined with questions.

    • Remove capacitors C61, C65 that are directly on inputs of the TLV9051. What is their purpose? Common mode filtering? If so, put them close to C64 instead. Capacitors across the input, or input to ground, reduce stability of the TLV9051. Refer to https://www.ti.com/lit/an/slyt087/slyt087.pdf
    • How far (physically) is the OPAMP TLV9051 from AMC1350? Both parts of the circuitry seem to be referred to GND. Are there ground shifts expected? The output of the AMC1350 is already differential. This is on purpose. Differential signaling mitigates common-mode transients. There is seldom need for additional analog filtering. Additionally, the nodes have different impedance. This means that the common mode filtering is different between the amplifier inputs. This makes things usually worse, not better. 
    • You filter the output of the AMC with R124, R129 and C64. This sets the corner frequency to 94kHz. Is this measure really necessary? The output bandwidth of the part is typ. 300kHz. If you insist on this filter, make sure that R124, R129 are close to TLV9051 and not to AMC1350.
    • You have C330pF in parallel with R135.  I assume this is an attempt to create a low pass filter. For difference amplifier, that both legs of the amplifier are symmetric. The exactly same capacitor has to be across the resistor R132. This is, in my opinion, where the problem comes from. 
    • How do you generate 1.5V voltage reference? It must be low-impedance source. 

    Please implement the changes and report back.

    Kind regards, 

    Jiri Panacek, Systems Applications Engineer

  • 0 in reply to Jiri Panacek85
    TI__Expert 4281 points

    edit: I see you generate the 1.5V reference from another TLV9051 channel. This is the right way to do it. Ignore my last bullet. :)