9

LT1113

S

APPLICATI

I FOR ATIO

This means the LT1113 will beat out any dual JFET op amp,

only the lowest noise bipolar op amps have the edge

(at low source resistances). As the source resistance

increases from 5k to 50k, the LT1113 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 LT1113 will out perform

the lowest noise bipolar op amp due to the inherently low

current noise of FET input op amps. Clearly, the LT1113

will extend the range of high impedance transducers

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

makes the LT1113 the best choice for high impedance,

capacitive transducers.

The high input impedance JFET front end makes the

LT1113 suitable in applications where very high charge

sensitivity is required. Figure 2 illustrates the LT1113 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

the input of the LT1113. The charge across the transducer

Figure 1. Comparison of LT1113 and LT1124 Total Output 1kHz Voltage Noise Versus Source Resistance

SOURCE RESISTANCE (

Ω)

100

1

10

1k

1k

100M

1113 • F01

100k

100

10M

10k

1M

RESISTOR NOISE ONLY

2

2

+

–

LT1124

LT1113

LT1124*

LT1113*

* PLUS RESISTOR

The LT1113 dual in the plastic and ceramic DIP packages

are pin compatible with and directly replace such JFET op

amps as the OPA2111 and OPA2604 with improved noise

performance. Being the lowest noise dual JFET op amp

available to date, the LT1113 can replace many bipolar op

amps that are used in amplifying low level signals from

high impedance transducers. The best bipolar op amps

will eventually loose out to the LT1113 when transducer

impedance increases due to higher current noise. The low

voltage noise of the LT1113 allows it to surpass every dual

and most single JFET op amps available. For the best

performance versus area available anywhere, the LT1113

is offered in the narrow SO-8 surface mount package with

standard pinout and no degradation in performance.

The low voltage and current noise offered by the LT1113

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 1 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.