AD745AN Analog Devices, AD745AN Datasheet - Page 8

• Manufacturer Part Number: AD745AN
• Description: Ultralow Noise/ High Speed/ BiFET Op Amp
• Manufacturer: Analog Devices

AD745
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
AD745. 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 10
T = Absolute Temperature, Kelvin (0 C = +273.2 Kelvin)
f = Bandwidth – in Hz (Assuming an Ideal “Brick Wall”
Filter)
Figure 27. Model for A High Z Follower with Gain
Figure 26. A Charge Amplifier Circuit
S
/C
R2
F
C
)) of the circuit.
S
C
S
C
B
S
B
*
be transferred to capacitor C
contributes a current noise of:
N
~
C *
R
R
B
B
B
*
R
*
R
B
B
4 k
*OPTIONAL, SEE TEXT
A1
R
C
T
F
A2
B
F
. The amplifiers input
R1
R 2
f
R1
R2
–23
R 1
=
C
Joules/Kelvin
C
S
F
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.
Figure 28. Noise at the Outputs of the Circuits of Figures
26 and 27. Gain = 10, C
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
minimize this resistor’s contribution to overall circuit noise.
When the equivalent current noise of R
the noise of I
R
Figure 29. Graph of Resistance vs. Input Bias Current
Where the Equivalent Noise
of the Bias Current I
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
to C
diminishing impact on noise; capacitor C
large mylar bypass capacitor of 0.01 F or greater.
B
larger.
S
–190
–100
–110
–120
–130
–140
–150
–160
–170
–180
–200
–210
–220
in Figure 27. At values of C
S
/C
0.01
F
5.2 x 10
5.2 x 10
= R1/ R2). One feature of the first circuit is that a
5.2 x 10
5.2 x 10
5.2 x 10
0.1
B
B
10
9
8
7
1pA
6
The value for C
2qI
1
B
B
FREQUENCY – Hz
, there is diminishing return in making
S
10pA
10
= 3000 pF, R
2qI
INPUT BIAS CURRENT
B
100
4 kT/R , Equals the Noise
B
100pA
B
in Figure 26 would be equal
over 300 pF, there is a
1k
B
B
(( 4 kT)/R) equals
B
= 22 M
1nA
can then be simply a
10k
B
B
large enough to
in Figures 26 and
100k
10nA
TOTAL OUTPUT
NOISE
NOISE DUE TO
R
NOISE DUE TO
I
B
B
ALONE
ALONE
REV. C
B
B