V62SLASH18609-01XE-R datasheet(16/29 Pages) TI1

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Frequency (Hz)

1000

10000

100000

1000000

1E+7

5E+7

100

105

D001

OPA2356-EP

SBOS955 – FEBRUARY 2019

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For additional information, see the Compensate transimpedance amplifiers intuitively application report.

8.2.1.2.2.1

Optimizing the Transimpedance Circuit

To achieve the best performance, select components according to the following guidelines:

1. For lowest noise, select R(FB) to create the total required gain. Using a lower value for R(FB) and adding gain

after the transimpedance amplifier generally produces poorer noise performance. The noise produced by

R(FB) increases with the square-root of R(FB), whereas the signal increases linearly. Therefore, signal-to-noise

ratio improves when all the required gain is placed in the transimpedance stage.

2. Minimize photodiode capacitance and stray capacitance at the summing junction (inverting input). This

capacitance causes the voltage noise of the op amp to amplify (increasing amplification at high frequency).

Using a low-noise voltage source to reverse-bias a photodiode can significantly reduce the capacitance.

Smaller photodiodes have lower capacitance. Use optics to concentrate light on a small photodiode.

3. Noise increases with increased bandwidth. Limit the circuit bandwidth to only that required. Use a capacitor

across the R(FB) to limit bandwidth, even if not required for stability.

4. Circuit board leakage can degrade the performance of an otherwise well-designed amplifier. Clean the circuit

board carefully. A circuit board guard trace that encircles the summing junction and is driven at the same

voltage can help control leakage.

For additional information, see the Noise analysis of FET transimpedance amplifiers and Noise analysis for high-

speed op amps application reports.

8.2.1.3 Application Curve

–3-dB bandwidth is 4.56 MHz

Figure 32. AC Transfer Function

8.2.2 High-Impedance Sensor Interface

Many sensors have high source impedances that may range up to 10 MΩ, or even higher. The output signal of

sensors often must be amplified or otherwise conditioned by means of an amplifier. The input bias current of this

amplifier can load the sensor output and cause a voltage drop across the source resistance, as shown in

Figure 33, where (V(+INx) = VS – I(BIAS) × R(S)). The last term, I(BIAS) × R(S), shows the voltage drop across R(S). To

prevent errors introduced to the system as a result of this voltage, an op amp with very low input bias current

must be used with high impedance sensors. This low current keeps the error contribution by I(BIAS) × R(S) less

than the input voltage noise of the amplifier, so that the input voltage noise does not become the dominant noise

factor. The OPA2356-EP op amp features very low input bias current (typically 200 fA) and is therefore a

preferred choice for such applications.

V62/18609-01XE-R Datasheet(PDF) 16 Page - Texas Instruments

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