AD745KRZ-16 Ver la hoja de datos (PDF) - Analog Devices
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AD745KRZ-16 Datasheet PDF : 12 Pages
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AD745
Figures 5 and 6 show two ways to buffer and amplify the output
of a charge output transducer. Both require the use of an ampli-
fier that has a very high input impedance, such as the AD745.
Figure 5 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 CS be transferred to capacitor CF, thus yielding an
output voltage of ∆Q/CF. The amplifiers input voltage noise will
appear at the output amplified by the noise gain (1 + (CS/CF))
of the circuit.
CF
RS
R1
R2
CS
A1
CB* RB*
R1 = CS
R2 CF
Figure 5. A Charge Amplifier Circuit
R1
CB*
R2
RB*
A2
CS RB
*OPTIONAL, SEE TEXT.
Figure 6. Model for A High Z Follower with Gain
The second circuit, Figure 6, is simply a high impedance fol-
lower with gain. Here the noise gain (1 + (R1/R2)) is the same
as the gain from the transducer to the output. Resistor RB, 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 RB contributes a current noise of:
~
N=
4 k T ∆f
RB
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 5 shows that these two circuits have an identical frequency
response and the same noise performance (provided that
CS/CF = R1/ R2). One feature of the first circuit is that a “T”
network is used to increase the effective resistance of RB and
improve the low frequency cutoff point by the same factor.
–100
–110
–120
–130
–140
–150
–160
–170
TOTAL
OUTPUT
NOISE
–180
–190
–200
–210
–220
0.01 0.1
1
10
100
1k
FREQUENCY – Hz
NOISE DUE TO
RB ALONE
NOISE DUE TO
IB ALONE
10k 100k
Figure 7. Noise at the Outputs of the Circuits of Figures 5
and 6. Gain = 10, CS = 3000 pF, RB = 22 MΩ
However, this does not change the noise contribution of RB
which, in this example, dominates at low frequencies. The graph
of Figure 8 shows how to select an RB large enough to minimize
this resistor’s contribution to overall circuit noise. When the
( ) equivalent current noise of RB ((ͱ4 kT)/R) equals the noise of
IB 2qIB , there is diminishing return in making RB larger.
5.2 ؋ 1010
5.2 ؋ 109
5.2 ؋ 108
5.2 ؋ 107
5.2 ؋ 106
1pA
10pA
100pA
1nA
INPUT BIAS CURRENT
10nA
Figure 8. Graph of Resistance vs. Input Bias Current
Where the Equivalent Noise ͙4 kT/R, Equals the Noise
( ) of the Bias Current IB 2qIB
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 RB in Figures 5 and 6.
As previously mentioned, for best noise performance care should
be taken to also balance the source capacitance designated by
CB The value for CB in Figure 5 would be equal to CS in
Figure 6. At values of CB over 300 pF, there is a diminishing
impact on noise; capacitor CB can then be simply a large mylar
bypass capacitor of 0.01 µF or greater.
–8–
REV. D
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