OPA2320 Datasheet, PDF (16/32 Pages) Texas Instruments – Precision, 20MHz, 0.9pA, Low-Noise, RRIO, CMOS Operational Amplifier with Shutdown

OPA320, OPA2320
OPA320S, OPA2320S
SBOS513D – AUGUST 2010 – REVISED NOVEMBER 2011
For single-supply applications, the +IN input can be
biased with a positive dc voltage to allow the output
to reach true zero when the photodiode is not
exposed to any light, and respond without the added
delay that results from coming out of the negative rail;
this configuration is shown in Figure 37. This bias
voltage also appears across the photodiode,
providing a reverse bias for faster operation.
(1)
C_F
< 1pF
R_F
10MW
V+
l
OPA320
V_OUT
+V_BIAS
(1) C_F is optional to prevent gain peaking. It includes the stray
capacitance of R_F.
Figure 37. Single-Supply Transimpedance
Amplifier
For additional information, refer to Application Bulletin
(SBOA055), Compensate Transimpedance Amplifiers
Intuitively, available for download at www.ti.com.
OPTIMIZING THE TRANSIMPEDANCE
CIRCUIT
To achieve the best performance, components should
be selected according to the following guidelines:
1. For lowest noise, select R_F to create the total
required gain. Using a lower value for R_F and
adding gain after the transimpedance amplifier
generally produces poorer noise performance.
The noise produced by R_F increases with the
square-root of R_F, 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 be amplified (increasing
amplification at high frequency). Using a
low-noise voltage source to reverse-bias a
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photodiode can significantly reduce its
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_F 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 Application
Bulletins Noise Analysis of FET Transimpedance
Amplifiers (SBOA060), and Noise Analysis for
High-Speed Op Amps (SBOA066), available for
download at the TI web site.
HIGH-IMPEDANCE SENSOR INTERFACE
Many sensors have high source impedances that
may range up to 10MΩ, 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 38, where (V_IN+ = V_S –
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 OPA320
series of op amps feature very low input bias current
(typically 200fA), and are therefore ideal choices for
such applications.
R_S
100kW
I_B
V_IN+
V+
OPA320
V-
V_OUT
R_F
R_G
Figure 38. Noise as a Result of I_BIAS
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