ad743jnz Analog Devices, Inc., ad743jnz Datasheet - Page 8

• Manufacturer Part Number: ad743jnz
• Description: Ultralow Noise Bifet Op Amp
• Manufacturer: Analog Devices, Inc.

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
voltage of ∆Q/C
the output amplified by the noise gain (1 + (C
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
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
where
k = Boltzman’s Constant = 1.381 × 10
T = Absolute Temperature, kelvin (0°C = 273.2 kelvin)
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
network is used to increase the effective resistance of R
improve the low frequency cutoff point by the same factor.
f = Bandwidth—in Hz (assuming an ideal “brick wall” filter)
S
Figure 5. Model for a High Z Follower with Gain
S
/C
be transferred to capacitor C
F
= R1/ R2). One feature of the first circuit is that a “T”
Figure 4. Charge Amplifier Circuit
F
B
. The amplifier’s input voltage noise will appear at
contributes a current noise of
R2
*OPTIONAL, SEE TEXT
C
S
*OPTIONAL, SEE TEXT
C
˜ N
B
*
= 4
C
R
C
R
B
B
S
B
R
*
*
*
B
A1
*
k
R
R
C
B
T
F
B
R2
A2
R1
R2
F
R1
R1
∆
, thus yielding an output
–23
f
=
C
C
joules/kelvin
S
F
S
/C
F
)) of the circuit.
B
and to
B
is
–8–
However, this does not change the noise contribution of R
in this example, dominates at low frequencies. The graph of
Figure 7 shows how to select an R
this resistor’s contribution to overall circuit noise. When the
equivalent current noise of R
(√2qIB), there is diminishing return in making R
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
As previously mentioned, for best noise performance, care should
be taken to also balance the source capacitance designated by C
The value for C
At values of C
noise; capacitor C
or greater.
Figure 6. Noise at the Outputs of the Circuits of
Figures 4 and 5. Gain = +10, C
Figure 7. Graph of Resistance vs. Input Bias Current
Where the Equivalent Noise √4kT/R , Equals the Noise
of the Bias Current √2qI
5.2
5.2
5.2
5.2
5.2
–100
–110
–120
–130
–140
–150
–160
–170
–180
–190
–200
–210
–220
10
10
10
10
10
10
0.01
9
8
7
6
1pA
B
B
over 300 pF, there is a diminishing impact on
0.1
in Figure 4 would be equal to C
B
can then be simply a large bypass of 0.01 µF
10pA
1
INPUT BIAS CURRENT
FREQUENCY (Hz)
B
B
((√4kT)/R equals the noise of I
10
R
DUE TO
B
100pA
B
NOISE
S
ALONE
large enough to minimize
= 3000 pF, R
100
I
B
DUE TO
NOISE
ALONE
B
1k
in Figures 4 and 5.
1nA
OUTPUT
TOTAL
B
NOISE
S
larger.
B
10k
in Figure 5.
= 22 M Ω
B
REV. E
100k
which,
10nA
B
B
.