ADA4511-2ARZ-R7 datasheet(23/28 Pages) AD

23 / 28 page

Data Sheet

ADA4511-2

APPLICATIONS INFORMATION

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Rev. A | 23 of 28

Figure 80. Error (LSB) vs. Switching Rate, 8-Channel 10 V Step

Figure 80 shows the error in least significant bit (LSB) vs. switching

rate for an 8-channel 10 V step mux data acquisition system. An

LSB error <1 is achieved up to 570 kHz switching rate.

TRANSIMPEDANCE AMPLIFIER

The ADA4511-2 is an excellent choice for low-noise transimpe-

dance amplifier (TIA) applications. The low voltage and current

noise of the ADA4511-2 maximize signal-to-noise ratio (SNR), and

the low VOS and IB of the ADA4511-2 minimize the DC error at the

amplifier output.

Common applications for current-to-voltage conversion include pho-

todiode circuits where the amplifier converts a current emitted

by a diode placed at the negative input terminal into an output

voltage. Some photodiode applications include fiber optic controls,

motion sensors, and barcode readers. The circuit shown in Figure

81 shows one channel of the ADA4511-2 as a current-to-voltage

converter with an electrical model of a photodiode.

Figure 81. Equivalent TIA Circuit

Photodiodes operate in either photovoltaic mode (zero bias) or

photoconductive mode (with an applied reverse-bias across the

diode). Mode selection depends on the speed and dark current

requirements of the application and the choice of photodiode. In

photovoltaic mode, the dark current is at a minimum and is prefer-

red for low-frequency and/or low-light level applications (that is, PN

photodiodes). Photoconductive mode is better for applications that

require faster and linear responses (that is, PIN photodiodes); how-

ever, the trade-offs include increases in dark and noise currents.

The following transfer function describes the transimpedance gain

of Figure 81:

VOUT = IDRF1+sCFRF

(2)

where:

VOUT is the required output DC voltage of the op amp.

ID is the output current of the photodiode.

RF is the feedback resistor.

CF is the feedback capacitor.

The parallel combination of RF and CF sets the signal bandwidth.

s is the complex frequency variable jω.

j is the imaginary unit.

ω is the angular frequency.

Set RF such that the maximum attainable VOUT corresponds to the

maximum diode IOUT. Because signal levels increase directly with

RF, while the noise due to RF increases with the square root of the

resistor value, which employs the full output swing maximizes the

SNR.

It is important to distinguish between the transimpedance gain and

the loop gain, because the loop gain characteristics determine the

net circuit stability. The closed-loop transfer function takes the form

shown in the following equation:

VOUTVIN

= A1+Aβ

(3)

where:

A is the open loop gain of the amplifier.

β is the feedback network.

Aβ is the loop gain.

In this application, β is given by the following:

β= RSHRSH+RF 1+sRFCF

1+sRF∥RSH CIN+CF

(4)

where:

RSH is the diode shunt resistance.

CIN is the total input capacitance consisting of the sum of the diode

shunt capacitance (CPD), the input capacitance of the amplifier

(CDM + CCM), and the external stray capacitance.

CIN, RF, CF, and RSH produce a zero in the 1/β transfer function.

The zero frequency (fZ) is as in the equation that follows:

fZ= 1

2π RF∥RSH CIN+CF

(5)

Because the photodiode shunt resistance RSH >> RF, the circuit

behavior is not impacted by the effect of the junction resistance,

and fZ simplifies to the following:

fZ= 1

2πRF(CIN+CF)

(6)

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