Data Sheet

ADA4352-2

analog.com

Rev 0

29 of 47

PGTIA ERRORS

PGTIA Measurements

The ADA4352-2 is designed for wide dynamic range transimpedance measurements with four internal gain resistors

of 315 Ω, 3.5 kΩ, 40.2 kΩ, and 450 kΩ. Additionally, each gain is internally compensated with feedback capacitors of

33 pF, 26.5 pF, 7 pF, and 2.5 pF, respectively. The dominant DC error sources will vary depending on the selected

PGTIA gain. When the TIA gain is set to 315 Ω, the majority of the DC error is caused by input offset voltage, while

when TIA gain is set to 450 kΩ, the error is dominated by input bias currents and switch off leakage currents.

The PGTIA circuit (see Figure 68) models a photodiode input as a parallel current source (IIN), resistor (RSH), and

capacitor (CD) at the inverting input of a closed-loop op amp. Through negative feedback, a virtual reference

voltage (VCM) is maintained at the inverting input terminal of the PGTIA. This potential sets the bias voltage for one

terminal of the photodiode and allows all the source current to flow through the feedback resistor. Ideally, only

signal current from the sensor must flow through the selected gain resistor of the PGTIA. The ideal transfer function

of the PGTIA is given by the following equation.

VOUT = Diode current (IIN) × RFx + VCM

where, RFx is one of RF0, RF1, RF2, or RF3.

In some cases, large photodiode reverse bias voltage (−VB) and/or elevated temperatures result in photodiode DC

dark current (IDARK), which can be considered both a DC error and current noise source in a photodiode model.

Figure 68. PGTIA Circuit

Input Offset Voltage

The input offset voltage of the amplifier in a PGTIA adds an output DC error (and drift) that is approximately the

same for all gain settings. The error at the output of the PGTIA due to the input offset is gained up by the (DC) noise

gain of the amplifier (see Figure 68).

Noise gain for a typical PGTIA is given by the following equation.

Noise gain = (1 + RFX/RSH)

where, RSH is any shunt resistance in the photodiode model.

ADA4352-2

GAIN RESISTOR NETWORK

KELVIN

SWITCH

NETWORK

OUT

–VB

½

VCM

CD

RSH

IIN

PHOTODIODE

=

RF2 = 40.2kΩ

RF3 = 450kΩ

RF0 = 315Ω

RF1 = 3.5kΩ