8

LT1792

S

APPLICATI

I FOR ATIO

best bipolar op amps, with higher current noise, will

eventually lose out to the LT1792 when transducer imped-

ance increases. The low voltage noise of the LT1792

allows it to surpass most single JFET op amps available.

For the best performance versus area available anywhere,

the LT1792 is offered in the SO-8 surface mount package

with no degradation in performance.

The low voltage and current noise offered by the LT1792

makes it useful in a wide range of applications, especially

where high impedance, capacitive transducers are used

such as hydrophones, precision accelerometers and photo

diodes. The total output noise in such a system is the gain

times the RMS sum of the op amp input referred voltage

noise, the thermal noise of the transducer, and the op amp

bias current noise times the transducer impedance.

Figure 2 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 LT1792 will beat out any JFET op amp, only

the lowest noise bipolar op amps have the edge

at low source resistances. As the source resistance in-

creases from 5k to 50k, the LT1792 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

high source resistances, the LT1792 will out perform

the lowest noise bipolar op amp due to the inherently low

Figure 3. Noninverting and Inverting Gain Configurations

R2

OUTPUT

R1

TRANSDUCER

OUTPUT

TRANSDUCER

1792 F03

Q = CV;

= I = C

dQ

dt

dV

dt

SOURCE RESISTANCE (

Ω)

100

1

10

1k

1k

100M

1792 F02

100k

100

10M

10k

1M

RESISTOR NOISE ONLY

2

2

+

–

LT1007

LT1792

LT1007*

LT1792*

* PLUS RESISTOR

 1000pF CAPACITOR

LT1792

†

LT1007

†

Figure 2. Comparison of LT1792 and LT1007 Total Output

1kHz Voltage Noise Versus Source Resistance

current noise of FET input op amps. Clearly, the LT1792

will extend the range of high impedance transducers

that can be used for high signal-to-noise ratios. This

makes the LT1792 the best choice for high impedance,

capacitive transducers.

The high input impedance JFET front end makes the

LT1792 suitable in applications where very high charge

sensitivity is required. Figure 3 illustrates the LT1792 in its

inverting and noninverting modes of operation. A charge

amplifier is shown in the inverting mode example; here the

gain depends on the principal of charge conservation at