Datasheet AD745 (Analog Devices) - 9

AD745
Figures 5 and 6 show two ways to buffer and amplify the output
–100
of a charge output transducer. Both require the use of an ampli-
–110
fier that has a very high input impedance, such as the AD745.
–120
Figure 5 shows a model of a charge amplifier circuit. Here,
–130 1V/ Hz
amplification depends on the principle of conservation of charge
O T –140
at the input of amplifier A1, which requires that the charge on
TOTAL –150 OUTPUT
capacitor C
NOISE
S be transferred to capacitor CF, thus yielding an
–160
output voltage of ∆Q/CF. The amplifiers input voltage noise will
–170
appear at the output amplified by the noise gain (1 + (CS/CF)) of the circuit.
–180 –190 NOISE DUE TO CF –200 DECIBELS REFERENCED RB ALONE –210 NOISE DUE TO RS R1 IB ALONE –220 0.01 0.1 1 10 100 1k 10k 100k R2 FREQUENCY – Hz
Figure 7. Noise at the Outputs of the Circuits of Figures 5 and 6. Gain = 10, C
A1
S = 3000 pF, RB = 22 MΩ
CS
However, this does not change the noise contribution of R
C
B
B* RB* R1 CS =
which, in this example, dominates at low frequencies. The graph
R2 CF
of Figure 8 shows how to select an RB large enough to minimize Figure 5. A Charge Amplifier Circuit this resistor’s contribution to overall circuit noise. When the equivalent current noise of RB ((冑4 kT)/R) equals the noise of
R1
I 2qI ( ) B B , there is diminishing return in making RB larger.
C B* 5.2 1010 RB* A2 R2 C R S B 5.2 109 *OPTIONAL, SEE TEXT.
Figure 6. Model for A High Z Follower with Gain
5.2 108 ANCE IN
The second circuit, Figure 6, is simply a high impedance fol- lower with gain. Here the noise gain (1 + (R1/R2)) is the same
RESIST
as the gain from the transducer to the output. Resistor R
5.2 107
B, 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,
5.2 1061pA 10pA 100pA 1nA 10nA
while resistor RB contributes a current noise of:
INPUT BIAS CURRENT
Figure 8. Graph of Resistance vs. Input Bias Current ~ T Where the Equivalent Noise 兹4 kT/R, Equals the Noise N = 4 k f ∆ R B of the Bias Current I 2qI ( ) B B where: To maximize dc performance over temperature, the source k = Boltzman’s Constant = 1.381 × 10–23 Joules/Kelvin resistances should be balanced on each input of the amplifier. T = Absolute Temperature, Kelvin (0°C = 273.2 Kelvin) This is represented by the optional resistor RB in Figures 5 and 6. ∆f = Bandwidth – in Hz (Assuming an Ideal “Brick Wall” As previously mentioned, for best noise performance care should be taken to also balance the source capacitance designated by Filter) CB The value for CB in Figure 5 would be equal to CS in This must be root-sum-squared with the amplifier’s own current Figure 6. At values of CB over 300 pF, there is a diminishing noise. impact on noise; capacitor CB can then be simply a large mylar Figure 5 shows that these two circuits have an identical frequency bypass capacitor of 0.01 µF or greater. 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. –8– REV. D