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AMC1200: Impedance matching for AMC1200 application circuit & Input filter Design & Ground signal connection

Part Number: AMC1200
Other Parts Discussed in Thread: TL431, LM4040

Hello all

I am going to design a motor phase current sampling circuit with AMC1200.

Q1: We know output signal of AMC1200 needs to be conditioned by op amp.Usually need a reference source to the input of the op amp.

So could you help me to confirm one thing :If I use TL431 as a reference source to connecte the input of the op amp and no voltage follower in the middle,then does it cause impedance matching problems?May I handle it in this way?

Q2:I found some input filter circuit has no resistor on VIN- pin. What is the difference between these two filters? What impact will it have?

 

Q3:How to connect the two signals VIN- and GND1(They are the same electrical net UHE_2 on my schematic).Can these two pins be connected with copper directly? What issues need to be considered about PCB layout ?

 

Looking forward to your help,Thanks

 

  • Hi Chao,

    I'll let the opamp team answer question 1.

    2. The reason for this extra resistor is to cancel the offset created by the input bias current of the device. This resistor should be the same size as the sense resistor, such that the same offset is created by the bias current flowing through the resistors connected to both the positive and negative inputs. 

    This excel calculator does not apply to the AMC1200 since the architecture is different, however it may help you visualize why this setup works: https://e2e.ti.com/support/amplifiers/f/14/p/815789/3019110#3019110

    3. These can be directly connected or after the "R3'" resistor as shown in the E2E link above. There is nothing particularly special that must be considered for PCB layout. I recommend looking at 10.2 in the datasheet or the EVM user's guide, located here for examples: http://www.ti.com/lit/ug/sbau187a/sbau187a.pdf

    Otherwise, simply make sure that the power supply, VDD1, is also referenced to the same GND. 

  • To add on, please change the C525 of the TL431 to a value that is outside of the instability region shown in fig 18 of the datasheet. If you must use a 1uF cap, i would recommend to change to a LM4040 2.5V variant as it is all cap stable.