AD743BQ Analog Devices, AD743BQ Datasheet - Page 8

• Manufacturer Part Number: AD743BQ
• Description: Ultralow Noise BiFET Op Amp
• Manufacturer: 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 C
yielding an output voltage of Q/C
voltage noise will appear at the output amplified by the noise
gain (1 + (C
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 R
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 R
where:
This must be root-sum-squared with the amplifier’s own current
noise.
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”
Figure 27. Model for a High Z Follower with Gain
Filter)
Figure 26. A Charge Amplifier Circuit
S
/C
B
F
)) of the circuit.
contributes a current noise of:
S
be transferred to capacitor C
~
N =
4k
R
T
F
B
. The amplifiers input
f
–23
Joules/Kelvin
F
, thus
B
,
–8–
Figure 28 shows that these two circuits have an identical
frequency response and the same noise performance (provided
that C
“T” network is used to increase the effective resistance of R
and improve the low frequency cutoff point by the same factor.
However, this does not change the noise contribution of R
which, in this example, dominates at low frequencies. The graph
of Figure 29 shows how to select an R
this resistor’s contribution to overall circuit noise. When the
equivalent current noise of 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
27. As previously mentioned, for best noise performance care
should be taken to also balance the source capacitance designated
by C
Figure 27. At values of C
impact on noise; capacitor C
of 0.01 F or greater.
( 2qI
Figure 28. Noise at the Outputs of the Circuits of Figures
26 and 27. Gain = 10, C
Figure 29. Graph of Resistance vs. Input Bias Current
where the Equivalent Noise 4kT/R , Equals the Noise
of the Bias Current
–100
–110
–120
–130
–140
–150
–160
–170
–180
–190
–200
–210
–220
B
. The value for C
S
10M
B
/C
), there is diminishing return in making R
F
5.2 x 10
5.2 x 10
5.2 x 10
5.2 x 10
5.2 x 10
= R1/ R2). One feature of the first circuit is that a
100M
10
7
6
9
8
1pA
1
B
FREQUENCY – Hz
in Figure 26 would be equal to C
B
10pA
2qI
10
S
over 300 pF, there is a diminishing
= 3000 pF, R
B
B
INPUT BIAS CURRENT
B
can then be simply a large bypass
(( 4kT)/R) equals the noise of I
100
100pA
B
1k
large enough to minimize
B
= 22 M
1nA
B
10k
in Figures 26 and
100k
B
larger.
TOTAL OUTPUT
NOISE
NOISE DUE TO
R
NOISE DUE TO
I
10nA
B
B
ALONE
ALONE
REV. C
S
B
, in
B
B