LT1793 Linear Technology, LT1793 Datasheet - Page 8

• Manufacturer Part Number: LT1793
• Description: Low Noise/ Picoampere Bias Current/ JFET Input Op Amp
• Manufacturer: Linear Technology

LT1793
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).
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
the gain times the RMS sum of the op amp’s input referred
8
A
PPLICATI
Figure 3. Comparison of LT1793 and LT1007 Total Output
1kHz Voltage Noise vs Source Resistance
100
10k
1k
10
1
100
SOURCE RESISTANCE = 2R
* PLUS RESISTOR
†
V
n
PLUS RESISTOR
= A
R
LT1793
–
+
S
O
C
1k
V
S
TRANSDUCER
RESISTOR NOISE ONLY
V
C
R
U
SOURCE RESISTANCE ( )
n
S
S
LT1007
10k
2
(OP AMP)
V
S
O
C
S
100k
I FOR ATIO
LT1793*
U
LT1007*
+ 4kTR + 2
1000pF CAPACITOR
R
LT1793
S
C
B
1M
S
= R
†
LT1007
10M
q
R
Figure 4. Inverting and Noninverting Gain Configurations
I
B
B
R
W
–
+
2
100M 1G
†
1793 F03
R
C
C
R
Q = CV;
F
F
B
B
= C
= R
F
F
dQ
dt
C
R
U
= I = C
S
S
OUTPUT
dV
dt
voltage noise, the thermal noise of the transducer, and the
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 (2qI
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.
Optimization Techniques for Charge Amplifiers
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 C
TRANSDUCER
R1
C
S
C
R
B
B
S
R
S
is transferred to the feedback capacitor C
R2
–
+
C
R
R
B
B
S
= R
> R1 OR R2
C
S
S
OUTPUT
1793 F04
B
R
2
) will eventually
F