TINA/Spice/LMH6554: ADC single-to-diff DC-coupled converter

Sergey Tikhonov

Part Number: LMH6554
Other Parts Discussed in Thread: TINA-TI,

Tool/software: TINA-TI or Spice Models

Hello,

I have a circuit with LMH6554 which used like a single to diff converter at the input of the ADC which has 1.25V common voltage requirements. So, I prepare schematic based on SNOA565A (www.ti.com/.../snoa565a.pdf). But in my case input signal has Vcm = 2V and it should be DC-coupled.
It is TINA schematic - LMH6554_single_to_diff.TSC- here I use not equal Rpu resistors in order to achieve output common voltage 1.25V for both outputs. Is it correct or not? And is there another option to achieve Vcmi = Vcmo?

Thank you!
Tsa

over 7 years ago

0 Rohit Bhat over 7 years ago

TI__Genius 15975 points

Hi,

Usually for level-shifting an input common mode (Vcm) voltage of 2V to an output Vcm of 1.25V, you would use a pull-down resistor to GND or a negative supply voltage. In your application, it looks like you had the output feedback pins connected incorrectly to the input pins which is why TINA-TI was giving incorrect DC voltages for equal value Rpu resistors.

I have attached a correct schematic of LMH6554 which would level-shift the input Vcm of 2V to an output Vcm of 1.25V with Vcmi = Vcmo. Only thing I added in your schematic is the 2V DC voltage on the un-driven input side. The choice of resistors for the gain setting of 2V/V automatically give Vcmi = Vcmo for an input Vcm of 2V and output common mode voltage of 1.25V. I have attached the TINA-TI schematic for the same.

7002.LMH6554_single_to_diff.TSC

Best Regards,

Rohit