Datasheet AD743 (Analog Devices) - 9

AD743
Figures 4 and 5 show two ways to buffer and amplify the output of
–100
a charge output transducer. Both require using an amplifier that
–110
has a very high input impedance, such as the AD743. Figure 4
–120 Hz
shows a model of a charge amplifier circuit. Here, amplifica-
/ TOTAL –130 OUTPUT
tion depends on the principle of conservation of charge at the
–140 NOISE
input of amplifier A1, which requires that the charge on capaci-
–150
tor CS be transferred to capacitor CF, thus yielding an output
–160
voltage of ∆Q/C
NOISE
F. The amplifier’s input voltage noise will appear at
–170 DUE TO
the output amplified by the noise gain (1 + (CS/CF)) of the circuit.
R –180 B ALONE –190 CF NOISE DECIBELS REFERENCED TO 1V –200 DUE TO RB* R1 I –210 B ALONE R2 –220 0.01 0.1 1 10 100 1k 10k 100k FREQUENCY (Hz) C A1 S
Figure 6. Noise at the Outputs of the Circuits of
C C R1 S
Figures 4 and 5. Gain = +10, C
B* R =
S = 3000 pF, RB = 22 MΩ
B* R2 CF
However, this does not change the noise contribution of RB which,
*OPTIONAL, SEE TEXT
in this example, dominates at low frequencies. The graph of Figure 4. Charge Amplifier Circuit Figure 7 shows how to select an RB large enough to minimize this resistor’s contribution to overall circuit noise. When the
R1
equivalent current noise of RB ((√4kT)/R equals the noise of IB (√2qIB), there is diminishing return in making R
C
B larger.
B* 5.2
ⴛ
1010 RB* A2 R2 CS RB 5.2
ⴛ
109 *OPTIONAL, SEE TEXT )
⍀ Figure 5. Model for a High Z Follower with Gain
5.2
ⴛ
108
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
RESISTANCE (
from the transducer to the output. In both circuits, resistor RB is
5.2
ⴛ
107
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
ⴛ
106
while resistor R
1pA 10pA 100pA 1nA 10nA
B contributes a current noise of
INPUT BIAS CURRENT
˜ T N = 4k f ∆ Figure 7. Graph of Resistance vs. Input Bias Current R Where the Equivalent Noise √4kT/R, Equals the Noise B of the Bias Current √2qIB 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 R B in Figures 4 and 5. f = Bandwidth—in Hz (assuming an ideal “brick wall” filter) As previously mentioned, for best noise performance, care should This must be root-sum-squared with the amplifier’s own be taken to also balance the source capacitance designated by CB. current noise. The value for CB in Figure 4 would be equal to CS in Figure 5. Figure 6 shows that these circuits in Figures 4 and 5 have an At values of CB over 300 pF, there is a diminishing impact on identical frequency response and noise performance (provided noise; capacitor CB can then be simply a large bypass of 0.01 µF that C or greater. S/CF = R1/ R2). One feature of the first circuit is that a “T” network is used to increase the effective resistance of RB and to improve the low frequency cutoff point by the same factor. –8– REV. E Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION PRODUCT HIGHLIGHTS CONNECTION DIAGRAMS SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ESD SUSCEPTIBILITY ORDERING GUIDE Typical Performance Characteristics OP AMP PERFORMANCE: JFET VS. BIPOLAR DESIGNING CIRCUITS FOR LOW NOISE LOW NOISE CHARGE AMPLIFIERS HOW CHIP PACKAGE TYPE AND POWER DISSIPATION AFFECT INPUT BIAS CURRENT REDUCED POWER SUPPLY OPERATION FOR LOWER IB AN INPUT IMPEDANCE COMPENSATED, SALLEN-KEY FILTER TWO HIGH PERFORMANCE ACCELEROMETER AMPLIFIERS LOW NOISE HYDROPHONE AMPLIFIER BALANCING SOURCE IMPEDANCES OUTLINE DIMENSIONS Revision History