Data Sheet

ADA4352-2

analog.com

Rev 0

31 of 47

For RF3 = 450 kΩ, the output-referred offset at 25°C is 150 µV, while the same circuit for RF0 = 315 Ω (lowest selectable

RFX value) has 130 µV of output-referred offset. In both cases, the output-referred offset is less than three ADC

codes. The 1.1 µV/°C (−40°C to +125°C) maximum offset temperature drift leads to a 181 µV shift or a worst case of

three additional codes.

The input bias current from the noninverting input (maximum 1 nA) has negligible impact on the TIA circuit

accuracy if it adds less than 10 µV (much less than one ADC code) of output-referred offset voltage error. This

occurs when the sensor output impedance (RSH) is less than 10 µV/1 nA = 10 kΩ.

If the ADA4352-2 is used in applications measuring a high impedance voltage sensor, the input bias current results

in a voltage error at the noninverting input. This error is equal to the sensor output impedance times the bias

current (see Figure 70). This voltage error is gained up by (1 + RFx/gain resistor (RG)).

Figure 70. High Impedance Voltage Measurement Using the ADA4352-2

Kelvin Connection and TIA Accuracy

A typical PGTIA places the switches in series with the feedback resistors (see Figure 71). Therefore, the switch on-

resistance is part of the transimpedance gain. To minimize the error, it is recommended to use gain setting

switches with the lowest possible on-resistance. Unfortunately, low impedance switches have large leakage at high

operating temperatures. They add leakage current to the sensitive signal path (see Switch Off Leakage Current). The

improved Kelvin approach used in the ADA4352-2 (see Figure 72) places half of the switches inside the loop to

provide a Kelvin connection. The on-resistance of the left side switch becomes a part of the open-loop output

impedance and is corrected by the loop gain of the amplifier (shown in Figure 72). As most PGTIA applications do

not need to drive DC current to their load, the on-resistance of the right-side switch does not create additional

error. The low on-resistance switches in ADA4352-2 are also designed for low leakage to minimize the error. For

traditional PGTIAs, the RON of the switches changes over temperature and causes gain error drift over temperature.

This error is more obvious for the smallest gain (315 Ω range). The ADA4352-2 Kelvin connection approach greatly

reduces this error.

ADA4352-2

GAIN RESISTOR NETWORK

KELVIN

SWITCH

NETWORK

OUT

½

RG

RSH

HIGH

IMPEDANCE

SENSOR

RF2 = 40.2kΩ

RF3 = 450kΩ

RF0 = 315Ω

RF1 = 3.5kΩ