Datasheet

R
100kW
S
V
IN+
I
B
OPA320
V+
V-
R
F
R
G
V
OUT
OPA320
V
OUT
V+
R
10MW
F
C
<1pF
F
(1)
l
+V
BIAS
OPA320, OPA2320
OPA320S, OPA2320S
www.ti.com SBOS513E –AUGUST 2010–REVISED JUNE 2013
For single-supply applications, the +IN input can be photodiode can significantly reduce its
biased with a positive dc voltage to allow the output capacitance. Smaller photodiodes have lower
to reach true zero when the photodiode is not capacitance. Use optics to concentrate light on a
exposed to any light, and respond without the added small photodiode.
delay that results from coming out of the negative rail;
3. Noise increases with increased bandwidth. Limit
this configuration is shown in Figure 41. This bias
the circuit bandwidth to only that required. Use a
voltage also appears across the photodiode,
capacitor across the R
F
to limit bandwidth, even if
providing a reverse bias for faster operation.
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
(1) C
F
is optional to prevent gain peaking. It includes the stray
otherwise conditioned by means of an amplifier. The
capacitance of R
F
.
input bias current of this amplifier can load the sensor
output and cause a voltage drop across the source
Figure 41. Single-Supply Transimpedance
resistance, as shown in Figure 42, where (V
IN+
= V
S
–
Amplifier
I
BIAS
× R
S
). The last term, I
BIAS
× R
S
, shows the
voltage drop across R
S
. To prevent errors introduced
For additional information, refer to Application Bulletin
to the system as a result of this voltage, an op amp
(SBOA055), Compensate Transimpedance Amplifiers
with very low input bias current must be used with
Intuitively, available for download at www.ti.com.
high impedance sensors. This low current keeps the
error contribution by I
BIAS
× R
S
less than the input
OPTIMIZING THE TRANSIMPEDANCE
voltage noise of the amplifier, so that it does not
CIRCUIT
become the dominant noise factor. The OPA320
series of op amps feature very low input bias current
To achieve the best performance, components should
(typically 200fA), and are therefore ideal choices for
be selected according to the following guidelines:
such applications.
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
Figure 42. Noise as a Result of I
BIAS
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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