8

LT1169

S

APPLICATI

I FOR ATIO

Figure 1. Comparison of LT1169, OP215, and AD822

Input Bias Current vs Common Mode Range

SOURCE RESISTANCE (

Ω)

100

1

10

1k

10k

1k

100M

1G

LT1169 • F02

100k

100

10M

10k

1M

2

2

* PLUS RESISTOR

RESISTOR NOISE ONLY

LT1169

LT1124*

LT1124

LT1169*

Figure 2. Comparison of LT1169 and LT1124 Total Output

1kHz Voltage Noise vs Source Resistance

the total noise. This means the LT1169 is superior to most

dual 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 LT1169

will match the best bipolar op amps for noise perfor-

mance, 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

nate the total noise. At these high source resistances, the

LT1169 will out perform the lowest noise bipolar op amps

due to the inherently low current noise of FET input op

amps. Clearly, the LT1169 will extend the range of high

impedance transducers that can be used for high signal-

to-noise ratios. This makes the LT1169 the best choice for

high impedance, capacitive transducers.

Optimization Techniques for Charge Amplifiers

The high input impedance JFET front end makes the

LT1169 suitable in applications where very high charge

sensitivity is required. Figure 3 illustrates the LT1169 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 LT1169. The charge across the transducer

should equal the transducer capacitance plus the input

In the noninverting mode example, the transducer current

is converted to a change in voltage by the transducer

LT1169 with a gain of 1 + R1/R2. A DC path is provided by

magnitude greater than the parallel combination of R1

currents, although small at room temperature, can create

significant errors over increasing temperature, especially

with transducer resistances of up to 1000M

Ω or more.

Amplifying Signals from High Impedance Transducers

The low voltage and current noise offered by the LT1169

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

voltage noise, the thermal noise of the transducer, and the

op amp’s input 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

COMMON MODE RANGE (V)

–15

–100

–60

–40

–20

0

20

40

–10

–5

05

LT1169 • F01

10

60

80

100

–80

15

LT1169

AD822

CURRENT NOISE =

OP215