R

100 kΩ

(S)

V

(+INx)

I

IB

OPA320-Q1

V

(V+)

V

(V–)

R

(F)

R

(G)

V

O

160

140

120

100

80

60

40

20

0

-20

1

100M

Frequency (Hz)

100k

1M

10M

10

100

1k

10k

0

-20

40

60

80

100

120

140

160

180

-

-

-

-

-

-

-

-

Phase

Gain

1000

100

10

1

10

1M

Frequency (Hz)

100

1k

10k

100k

19

OPA320-Q1, OPA2320-Q1

www.ti.com

SLOS884B – SEPTEMBER 2014 – REVISED DECEMBER 2018

Product Folder Links: OPA320-Q1 OPA2320-Q1

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Copyright © 2014–2018, Texas Instruments Incorporated

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, refer to the following documents:

•

Texas Instruments, Noise Analysis of FET Transimpedance Amplifiers Application Bulletin

•

Texas Instruments, Noise Analysis for High-Speed Op Amps Application Report

8.2.1.3 Application Curves

Wide gain bandwidth as shown in Figure 38 and low input voltage noise as shown in Figure 39 make the

OPAx320-Q1 device an excellent wideband photodiode transimpedance amplifier.

VS = ±2.5 V, C(L) = 50 pF

Figure 38. Open-Loop Gain and Phase vs Frequency

VS = 1.8 to 5.5 V

Figure 39. Input Voltage Noise Spectral Density vs

Frequency

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 40, 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 it does not become the dominant noise factor. The

OPAx320-Q1 series of op amps feature very low input bias current (typically 200 fA), and are therefore excellent

choices for such applications.

Figure 40. Noise as a Result of I(BIAS)