LMH32401: Rin specification

AJ Coetzee

Prodigy 50 points

Part Number: LMH32401
Other Parts Discussed in Thread: TINA-TI,

Concerning input resistance specification for 20k gain

INPUT PERFORMANCE
RIN Input Resistance 270 350 410 Ω.

I want to use the device with a detector that will sink current from the input pin.

The detector has a large capacitance and a large bulk resistance (20pF/300R).

I would like to know exactly what the specification for Rin indicates, because if the input impedance is 350R, bandwidth will be seriously limited.

Measurements on the development board (with 2k connected to the input) indicates a gain of 10 as expected, and the additional 350R does not seem to have an effect.

over 3 years ago

0 kai klaas69 over 3 years ago

Guru 140200 points

Hi Aj,

why not running a simulation with the LMH32401 TINA-TI reference design?

LMH32401 TINA-TI Model (Rev. B)

Kai

0 AJ Coetzee over 3 years ago in reply to kai klaas69

Prodigy 50 points

Hi Kai

I am using the cadence pspice simulator and have drawn the evaluation board schematic and simulated that.

The DC values at Vocm (1.125V HW vs 1.65V sim) and the outputs (OUT+: 3.23V sim vs 0.821 HW: OUT-: 3.23V sim vs 1.43 HW: )differ for the simulation and the measured values on the evaluation board hardware.

The AC simulation also shows a difference for 30mV RMS input to the evaluation board. Differential output on the hardware is exactly 300mV rms as expected, but the simulation shows 250mV @ 1MHz.

I would still like to know what the Rin specification on the datasheet means, since it was important enough to the designer of the chip to make that value available.

I can send the simulation files, if you can send me an email address.