廬
APPLICATION BULLETIN
Mailing Address: PO Box 11400 鈥?Tucson, AZ 85734 鈥?Street Address: 6730 S. Tucson Blvd. 鈥?Tucson, AZ 85706
Tel: (602) 746-1111 鈥?Twx: 910-952-111 鈥?Telex: 066-6491 鈥?FAX (602) 889-1510 鈥?Immediate Product Info: (800) 548-6132
IMPROVED NOISE PERFORMANCE OF THE
ACF2101 SWITCHED INTEGRATOR
By Bonnie C. Baker (602) 746-7984
The signal-to-noise ratio and bandwidth of the combination
of the ACF2101 dual, switched integrator and a low-level
input current is exceptional when compared to the perfor-
mance of a classical transimpedance amplifier (Figure 1). To
further improve the ACF2101 signal-to-noise ratio, a resis-
tor can be added in series with the input sensor.
Where:
V
OUT
= output voltage of op amp
C
INT
= integration capacitor
I
IN
= sensor current
The output of the ACF2101 switched integrator is a time
averaged representation of the input.
C
INT
1/2 ACF2101
Reset
V
OUT
Sensor
I
IN
Hold
C
INT
V
OUT
Integrator Amplifier
Sensor
R
2
V
OUT
Sensor
I
IN
FIGURE 2. The ACF2101 Switched Integrator Block Dia-
gram.
Once the ACF2101 has integrated the input signal over a
predetermine period, the HOLD switch is opened, allowing
the user to read the output of the switched integrator at a held
voltage. The HOLD switch performs a sample/hold function
on the signal. Once the signal is read, the RESET switch is
closed in order to discharge the integration capacitor, C
INT
,
and bring the output back to the same potential as the
inverting input of the amplifier. Once the output returns to
ground, the RESET switch is opened. Shortly after the
RESET switch is opened, the HOLD switch closes to start
the integration cycle again.
Typically, a photodiode is used as the sensor for both
circuits shown in Figure 1. A photodiode can be modeled
using the sensor model shown in Figure 3. This model
includes a current source (I
IN
), parasitic resistor (R
1
), and
parasitic capacitor (C
1
). Typical values of R
1
range from
100k鈩?to 100G鈩? Typical values of C
1
range from 20pF to
1000pF. C
1
can be higher if the sensor is placed at a remote
location, and a cable, with parasitic capacitance to ground,
is used to transmit the signal to the input of the switched
integrator.
AB-053
Printed in U.S.A. May, 1993
Classical Transimpedance Amplifier
FIGURE 1. Typical Circuits Used to Convert Current Sig-
nals to Voltage.
The ACF2101 is a dual switched integrator, as shown in
Figure 2. The current from the sensor is integrated by the
capacitor (C
INT
) in the feedback loop of the amplifier. Since
the inverting input of the amplifier is kept at a virtual
ground, the output of the integrator changes in a negative
direction over time. The resulting transfer function of the
switched integrator is:
t
V
OUT
= 鈥?
鈭?/div>
O
I
IN
dt
C
INT
漏
1993 Burr-Brown Corporation
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