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OPA354-Q1_15 Datasheet, PDF (20/36 Pages) Texas Instruments – OPAx354-Q1 250-MHz, Rail-to-Rail I/O, CMOS Operational Amplifiers
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OPA354-Q1, OPA2354-Q1, OPA4354-Q1
SBOS492B – JUNE 2009 – REVISED DECEMBER 2014
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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, refer to the following application bulletins from TI: Noise Analysis of FET
Transimpedance Amplifiers (SBOA060), and Noise Analysis for High-Speed Op Amps (SBOA066).
8.2.1.3 Application Curve
105
100
95
90
85
80
75
70
65
60
1000
10000
100000 1000000
Frequency (Hz)
1E+7
5E+7
D001
–3 dB bandwidth is 4.56 MHz
Figure 39. 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 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
OPAx354-Q1 family of devices series of op amps feature very low input bias current (typically 200 fA), and are
therefore ideal choices for such applications.
R(S)
100 kΩ
IIB V(+INx)
V(V+)
Device
V(V–)
VO
R(F)
R(G)
Figure 40. Noise as a Result of I(BIAS)
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