Datasheet

REV. E–8–
AD743
Figures 4 and 5 show two ways to buffer and amplify the output of
a charge output transducer. Both require using an amplifier that
has a very high input impedance, such as the AD743. Figure 4
shows a model of a charge amplifier circuit. Here, amplifica-
tion depends on the principle of conservation of charge at the
input of amplifier A1, which requires that the charge on capaci-
tor C
S
be transferred to capacitor C
F
, thus yielding an output
voltage of ∆Q/C
F
. The amplifier’s input voltage noise will appear at
the output amplified by the noise gain (1 + (C
S
/C
F
)) of the circuit.
A1
*OPTIONAL, SEE TEXT
C
S
C
F
C
B
*
R
B
*
R1
=
C
S
C
F
R1
R2
R2
R
B
*
Figure 4. Charge Amplifier Circuit
A2
*OPTIONAL, SEE TEXT
C
S
C
B
*
R
B
*
R1
R2
R
B
Figure 5. Model for a High Z Follower with Gain
The circuit in Figure 5 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. In both circuits, resistor R
B
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 R
B
contributes a current noise of
˜
Nk
T
R
f
B
= 4 ∆
where
k = Boltzman’s Constant = 1.381 × 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.
Figure 6 shows that these circuits in Figures 4 and 5 have an
identical frequency response and noise performance (provided
that C
S
/C
F
= R1/ R2). One feature of the first circuit is that a “T”
network is used to increase the effective resistance of R
B
and to
improve the low frequency cutoff point by the same factor.
–100
–110
–120
–130
–140
–150
–160
–170
–180
–190
–200
–210
–220
0.01
0.1
110100
1k
10k 100
k
FREQUENCY (Hz)
DECIBELS REFERENCED TO 1V/
Hz
TOTAL
OUTPUT
NOISE
NOISE
DUE TO
R
B
ALONE
NOISE
DUE TO
I
B
ALONE
Figure 6. Noise at the Outputs of the Circuits of
Figures 4 and 5. Gain = +10, C
S
= 3000 pF, R
B
= 22 M Ω
However, this does not change the noise contribution of R
B
which,
in this example, dominates at low frequencies. The graph of
Figure 7 shows how to select an R
B
large enough to minimize
this resistor’s contribution to overall circuit noise. When the
equivalent current noise of R
B
((√4kT)/R equals the noise of I
B
(√2qIB), there is diminishing return in making R
B
larger.
1pA 10pA 100pA 1nA 10nA
5.2 ⴛ 10
10
5.2 ⴛ 10
9
5.2 ⴛ 10
7
5.2 ⴛ 10
6
5.2 ⴛ 10
8
INPUT BIAS CURRENT
RESISTANCE (⍀)
Figure 7. Graph of Resistance vs. Input Bias Current
Where the Equivalent Noise
√4kT/R , Equals the Noise
of the Bias Current √2qI
B
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 R
B
in Figures 4 and 5.
As previously mentioned, for best noise performance, care should
be taken to also balance the source capacitance designated by C
B
.
The value for C
B
in Figure 4 would be equal to C
S
in Figure 5.
At values of C
B
over 300 pF, there is a diminishing impact on
noise; capacitor C
B
can then be simply a large bypass of 0.01 µF
or greater.