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OPA320AIDBVT Datasheet(PDF) 17 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor. Click here to check the latest version.
Part No. OPA320AIDBVT
Description  Precision, 20MHz, 0.9pA, Low-Noise, RRIO, CMOS Operational Amplifier with Shutdown
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Maker  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com
Logo NSC - National Semiconductor (TI)

OPA320AIDBVT Datasheet(HTML) 17 Page - National Semiconductor (TI)

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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 RF 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 10M
Ω, or even higher. The output
signal
of
sensors
often
must
be
amplified
or
(1) CF is optional to prevent gain peaking. It includes the stray
otherwise conditioned by means of an amplifier. The
capacitance of RF.
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 (VIN+ = VS
Amplifier
IBIAS × RS). The last term, IBIAS × RS, shows the
voltage drop across RS. 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 IBIAS × RS 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 RF to create the total
required gain. Using a lower value for RF and
adding gain after the transimpedance amplifier
generally produces poorer noise performance.
The noise produced by RF increases with the
square-root of RF, 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 IBIAS
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
Copyright © 2010–2013, Texas Instruments Incorporated
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17
Product Folder Links: OPA320 OPA2320 OPA320S OPA2320S


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