LT1793
PPLICATI
with temperature will occur when the device is nulled with
a potentiometer ranging from 10k to 200k. Finer adjust-
ments can be made with resistors in series with the
potentiometer (Figure 2b).
W
U
O U
I FOR ATIO
S
A
voltagenoise, thethermalnoiseofthetransducer, andthe
op amp’s input bias current noise times the transducer
impedance. Figure 3 shows total input voltage noise
versus source resistance. In a low source resistance
(<5k) application the op amp voltage noise will dominate
the total noise. This means the LT1793 is superior to
most JFET op amps. Only the lowest noise bipolar op
amps have the advantage at low source resistances. As
the source resistance increases from 5k to 50k, the
LT1793 will match the best bipolar op amps for noise
performance, since the thermal noise of the transducer
(4kTR) begins to dominate the total noise. A further
increase in source resistance, above 50k, is where the op
amp’s current noise component (2qIBR2) will eventually
dominate the total noise. At these high source resis-
tances, the LT1793 will out perform the lowest noise
bipolar op amps due to the inherently low current noise of
FET input op amps. Clearly, the LT1793 will extend the
range of high impedance transducers that can be used for
high signal-to-noise ratios. This makes the LT1793 the
best choice for high impedance, capacitive transducers.
Amplifying Signals from High Impedance Transducers
The low voltage and current noise offered by the LT1793
makes it useful in a wide range of applications, especially
where high impedance, capacitive transducers are used
such as hydrophones, precision accelerometers and
photodiodes. The total output noise in such a system is
thegaintimestheRMSsumoftheopamp’sinputreferred
10k
C
LT1007*
S
–
+
R
LT1793*
S
1k
100
10
V
O
R
C
S
S
LT1007†
LT1793
LT1793†
LT1007
RESISTOR NOISE ONLY
Optimization Techniques for Charge Amplifiers
1
The high input impedance JFET front end makes the
LT1793 suitable in applications where very high charge
sensitivity is required. Figure 4 illustrates the LT1793 in its
inverting and noninverting modes of operation. A charge
amplifier is shown in the inverting mode example; the gain
depends on the principal of charge conservation at the
input of the LT1793. The charge across the transducer
capacitance CS is transferred to the feedback capacitor CF
100
1k 10k 100k 1M 10M 100M 1G
SOURCE RESISTANCE (Ω)
1793 F03
SOURCE RESISTANCE = 2R = R
S
* PLUS RESISTOR
†
PLUS RESISTOR
1000pF CAPACITOR
2
2
V = A √V
n
+ 4kTR + 2qI R
B
V
n (OP AMP)
Figure 3. Comparison of LT1793 and LT1007 Total Output
1kHz Voltage Noise vs Source Resistance
R
R2
F
C
B
C
F
R
B
–
+
–
+
R
C
OUTPUT
R1
OUTPUT
S
S
C
= C
C
S
S
B
B
F
F
TRANSDUCER
R
= R
R
R
C
S
C
R
R
C
S
S
dQ
Q = CV; = I = C
dt
dV
dt
B
B
S
= R
S
R
B
C
B
> R1 OR R2
TRANSDUCER
1793 F04
Figure 4. Inverting and Noninverting Gain Configurations
8
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