Datasheet
REV. E
AD743
–11–
R2
1900⍀
R4*
R1
10
8
⍀
R3
100⍀
OUTPUT
AD743
*OPTIONAL, SEE TEXT
INPUT SENSITIVITY = –179 dB re. 1V/Pa**
**1V PER MICROPASCAL
B AND K TYPE 8100
HYDROPHONE
C
T
C
C
C1*
Figure 15b. AC-Coupled, Low Noise
Hydrophone Amplifier
R5
100k⍀
R1
10
8
⍀
R4*
10
8
⍀
OUTPUT
DC OUTPUT 1mV FOR I
B
(AD743) 100nA
C
T
C1*
AD743
R2
1900⍀
R6
1M⍀
R7
16M⍀
16M⍀
*OPTIONAL, SEE TEXT
C2
0.27F
R3
100⍀
B AND K
TYPE 8100
HYDROPHONE
AD711K
Figure 15c. Hydrophone Amplifier Incorporating a
DC Servo Loop
where the dc gain is 1 and the gain above the low frequency cutoff
(1/(2πC
C
(100 Ω))) is the same as the circuit of Figure 15a. The
circuit of Figure 15c uses a dc servo loop to keep the dc output
at 0 V and to maintain full dynamic range for I
B
up to 100 nA.
The time constant of R7 and C2 should be larger than that of
R1 and C
T
for a smooth low frequency response.
The transducer shown has a source capacitance of 7500 pF. For
smaller transducer capacitances (≤300 pF), the lowest noise can
be achieved by adding a parallel RC network (R4 = R1, C1 = C
T
)
in series with the inverting input of the AD743.
BALANCING SOURCE IMPEDANCES
As mentioned previously, it is good practice to balance the
source impedances (both resistive and reactive) as seen by the
inputs of the AD743. Balancing the resistive components will
optimize dc performance over temperature because balancing
will mitigate the effects of any bias current errors. Balancing
input capacitance will minimize ac response errors due to the
amplifier’s input capacitance and, as shown in Figure 16, noise
performance will be optimized. Figure 17 shows the required
external components for noninverting (A) and inverting (B)
configurations.
INPUT CAPACITORS (pF)
RTI VOLTAGE NOISE (nV/√Hz)
40
30
20
10
10 100 1000
UNBALANCED
BALANCED
2.9nV/√Hz
Figure 16. RTI Voltage Noise vs. Input Capacitance
OUTPUT
R1
R2
R
B
C
B
C
S
R
S
NONINVERTING
CONNECTION
A
A
C
B
= C
S
R
B
= R
S
FOR
R
S
>> R1 OR R2
OUTPUT
R1
C
S
C
F
C
B
R
S
R
B
INVERTING
CONNECTION
B
B
C
B
= C
F
储 C
S
R
B
= R1 储 R
S
Figure 17. Optional External Components for Balancing Source Impedances