ad743jnz Analog Devices, Inc., ad743jnz Datasheet - Page 10

• Manufacturer Part Number: ad743jnz
• Description: Ultralow Noise Bifet Op Amp
• Manufacturer: Analog Devices, Inc.

AD743
AN INPUT IMPEDANCE COMPENSATED, SALLEN-KEY
FILTER
The simple high-pass filter of Figure 13 has an important source
of error which is often overlooked. Even 5 pF of input capacitance
in amplifier A will contribute an additional 1% of pass-band ampli-
tude error, as well as distortion, proportional to the C/V characteristics
of the input junction capacitance. The addition of the network
designated Z will balance the source impedance—as seen by
A—and thus eliminate these errors.
TWO HIGH PERFORMANCE ACCELEROMETER
AMPLIFIERS
Two of the most popular charge-out transducers are hydrophones
and accelerometers. Precision accelerometers are typically cali-
brated for a charge output (pC/g).* Figures 14a and 14b show
two ways in which to configure the AD743 as a low noise charge
amplifier for use with a wide variety of piezoelectric accelerom-
eters. The input sensitivity of these circuits will be determined
by the value of capacitor C1 and is equal to
The ratio of capacitor C1 to the internal capacitance (C
transducer determines the noise gain of this circuit (1 + C
The amplifier’s voltage noise will appear at its output amplified
by this amount. The low frequency bandwidth of these circuits
will be dependent on the value of resistor R1. If a T network is
used, the effective value is R1(1 + R2/R3).
1000pF
500k
Figure 13. Input Impedance Compensated
Sallen-Key Filter
1000pF
Figure 14a. Basic Accelerometer Circuit
B AND K MODEL
EQUIVALENT
4370 OR
Z
500k
*pC = PICOCOULOMBS
g = EARTH’S GRAVITATIONAL CONSTANT
∆
(5
V
A
110M
OUT
+V
–V
R1
22M )
S
S
1250pF
=
C1
AD743
∆
R3
1k
Z
Q
C
9k
R2
OUT
1
1000pF
1000pF
OUTPUT
0.8mV/pC*
500k
T
500k
) of the
T
/C1).
–10–
A dc servo loop (Figure 14b) can be used to assure a dc output
which is <10 mV, without the need for a large compensating
resistor when dealing with bias currents as large as 100 nA. For
optimal low frequency performance, the time constant of the
servo loop (R4C2 = R5C3) should be
LOW NOISE HYDROPHONE AMPLIFIER
Hydrophones are usually calibrated in the voltage out mode.
The circuits of Figures 15a and 15b can be used to amplify the
output of a typical hydrophone. Figure 15a shows a typical
dc-coupled circuit. The optional resistor and capacitor serve
to counteract the dc offset caused by bias currents flowing through
resistor R1. Figure 15b, a variation of the original circuit, has a
low frequency cutoff determined by an RC time constant equal to
Figure 14b. Accelerometer Circuit Using a DC
Servo Amplifier
B AND K TYPE 8100
B AND K MODEL
Figure 15a. Basic Hydrophone Amplifier
EQUIVALENT
HYDROPHONE
4370 OR
Time Cons
**1V PER MICROPASCAL
INPUT SENSITIVITY = –179 dB re. 1V/ Pa**
*OPTIONAL, SEE TEXT
Time Cons
(5
100
110M
R3
R1
C
22M )
1k
T
R3
AD711
tan =
tant ≥
2.2 F
10
10
C1*
R4*
t
C2
1250pF
R1
8
8
AD743
C1
10 1 1
2
π
C3
2.2 F
R
AD743
18M
18M
1900
×
R4
R5
9k
R2
R2
C



C
1
+
×
R
R
100
2
3



C
Ω
OUTPUT
0.8mV/pC
1
OUTPUT
REV. E