8

LT1793

S

APPLICATI

I FOR ATIO

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

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

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

Figure 3. Comparison of LT1793 and LT1007 Total Output

1kHz Voltage Noise vs Source Resistance

Figure 4. Inverting and Noninverting Gain Configurations

SOURCE RESISTANCE (

Ω)

100

1

10

1k

10k

1k

100M

1G

1793 F03

100k

100

10M

10k

1M

2

2

* PLUS RESISTOR

 1000pF CAPACITOR

RESISTOR NOISE ONLY

LT1793

LT1007*

LT1007

†

LT1793

†

LT1007

LT1793*

R2

OUTPUT

R1

TRANSDUCER

OUTPUT

TRANSDUCER

1793 F04

Q = CV;

= I = C

dQ

dt

dV

dt