AD743JR-16-REEL(2003) 查看數據表（PDF） - Analog Devices

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AD743JR-16-REEL
(Rev.:2003)

Ultralow Noise BiFET Op Amp

Analog Devices 

AD743

Figures 26 and 27 show two ways to buffer and amplify the

output of a charge output transducer. Both require using an

amplifier which has a very high input impedance, such as the

AD743. Figure 26 shows a model of a charge amplifier circuit.

Here, amplification depends on the principle of conservation of

charge at the input of amplifier A1, which requires that the

charge on capacitor CS be transferred to capacitor CF, thus

yielding an output voltage of ∆Q/CF. The amplifiers input

voltage noise will appear at the output amplified by the noise

gain (1 + (CS/CF)) of the circuit.

Figure 26. A Charge Amplifier Circuit

Figure 28 shows that these two circuits have an identical

frequency response and the same noise performance (provided

that CS/CF = R1/ R2). One feature of the first circuit is that a

“T” network is used to increase the effective resistance of RB

and improve the low frequency cutoff point by the same factor.

–100

–110

–120

–130

–140

–150

–160

–170

–180

–190

–200

–210

–220

10M

100M

1

10

100

1k

FREQUENCY – Hz

TOTAL OUTPUT

NOISE

NOISE DUE TO

R B ALONE

NOISE DUE TO

IB ALONE

10k 100k

Figure 28. Noise at the Outputs of the Circuits of Figures

26 and 27. Gain = 10, CS = 3000 pF, RB = 22 MΩ

However, this does not change the noise contribution of RB

which, in this example, dominates at low frequencies. The graph

of Figure 29 shows how to select an RB large enough to minimize

this resistor’s contribution to overall circuit noise. When the

equivalent current noise of RB ((√4kT)/R) equals the noise of IB

( 2qIB ), there is diminishing return in making RB larger.

5.2 x 1010

5.2 x 10 9

5.2 x 10 8

Figure 27. Model for a High Z Follower with Gain

The second circuit, Figure 27, is simply a high impedance

follower with gain. Here the noise gain (1 + (R1/R2)) is the

same as the gain from the transducer to the output. Resistor RB,

in both circuits, is required as a dc bias current return.

There are three important sources of noise in these circuits.

Amplifiers A1 and A2 contribute both voltage and current noise,

while resistor RB contributes a current noise of:

~

N=

T

4k

∆f

RB

where:

k = Boltzman’s Constant = 1.381 x 10–23 Joules/Kelvin

T = Absolute Temperature, Kelvin (0°C = +273.2 Kelvin)

∆f = Bandwidth – in Hz (Assuming an Ideal “Brick Wall”

Filter)

This must be root-sum-squared with the amplifier’s own current

noise.

5.2 x 107

5.2 x 10 6

1pA

10pA

100pA

1nA

INPUT BIAS CURRENT

10nA

Figure 29. Graph of Resistance vs. Input Bias Current

where the Equivalent Noise √4kT/R, Equals the Noise

of the Bias Current 2qIB

To maximize dc performance over temperature, the source

resistances should be balanced on each input of the amplifier.

This is represented by the optional resistor RB in Figures 26 and

27. As previously mentioned, for best noise performance care

should be taken to also balance the source capacitance designated

by CB. The value for CB in Figure 26 would be equal to CS, in

Figure 27. At values of CB over 300 pF, there is a diminishing

impact on noise; capacitor CB can then be simply a large bypass

of 0.01 µF or greater.

–8–

REV. C

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