AD743

REV. C

–8–

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

yielding an output voltage of

∆Q/C

voltage noise will appear at the output amplified by the noise

Figure 26. A Charge Amplifier Circuit

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

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,

~

N =

4k

T

∆f

where:

k = Boltzman’s Constant = 1.381 x 10

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.

Figure 28 shows that these two circuits have an identical

frequency response and the same noise performance (provided

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

10k

100k

FREQUENCY – Hz

TOTAL OUTPUT

NOISE

NOISE DUE TO

R

ALONE

B

NOISE DUE TO

I

ALONE

B

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

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

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

√4kT)/R) equals the noise of I

B

( 2qI

1pA

10pA

100pA

1nA

10nA

5.2 x 10

10

5.2 x 10

9

5.2 x 10

8

5.2 x 10

7

5.2 x 10

6

INPUT BIAS CURRENT

Figure 29. Graph of Resistance vs. Input Bias Current

where the Equivalent Noise

√4kT/R, Equals the Noise

of the Bias Current

To maximize dc performance over temperature, the source

resistances should be balanced on each input of the amplifier.

27. As previously mentioned, for best noise performance care

should be taken to also balance the source capacitance designated

of 0.01

µF or greater.