LT1793 Datasheet, PDF(8/12 Page) Linear Technology – Low Noise, Picoampere Bias Current, JFET Input Op Amp

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LT1793 Datasheet, PDF (8/12 Pages) Linear Technology – Low Noise, Picoampere Bias Current, JFET Input Op Amp

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LT1793

APPLICATI S I FOR ATIO

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 ampâs input referred

10k

LT1007*

â RS

LT1793*

100

RS CS

LT1007â

LT1793

LT1793â

LT1007

RESISTOR NOISE ONLY

100 1k 10k 100k 1M 10M 100M 1G

SOURCE RESISTANCE (â¦)

1793 F03

SOURCE RESISTANCE = 2RS = R

* PLUS RESISTOR

â PLUS RESISTOR ï£¦ï£¦ 1000pF CAPACITOR

Vn = AV âVn2(OP AMP) + 4kTR + 2qIBR2

Figure 3. Comparison of LT1793 and LT1007 Total Output

1kHz Voltage Noise vs Source Resistance

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

op ampâs 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

ampâs current noise component (2qIBR2) will eventually

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 CS is transferred to the feedback capacitor CF

CS RS

TRANSDUCER

OUTPUT

CB = CFï£´ï£´CS

RB = RFï£´ï£´RS

CV;ddQt

OUTPUT

CS RS

TRANSDUCER

CB â CS

RB = RS

RS > R1 OR R2

1793 F04

Figure 4. Inverting and Noninverting Gain Configurations

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