AD822
Figure 46 shows a method for extending capacitance load drive
capability for a unity-gain followerꢀ With these component
values, the circuit drives 5000 pF with a ±0% overshootꢀ
20mV
. . . . . . . .
2µs
. . . .
. . . . . . . . . . . . . . . .
. . . . . . . . . . . .
100
90
V+
0.01µF
8
+
1/2
100Ω
V
IN
AD822
V
OUT
–
0.01µF
20pF
C
4
L
10
. . . . . . . .
. . . .
. . . . . . . . . . . .
. . . . . . . . . . . . . . . .
0%
V–
20kΩ
Figure 46. Extending Unity-Gain Follower Capacitive Load Capability
Beyond 350 pF
Figure 44. Small Signal Response of AD822 as
Unity-Gain Follower Driving 350 pF
Figure 45 is a plot of noise gain vsꢀ capacitive load that results in
a 20° phase margin for the AD822ꢀ Noise gain is the inverse of
the feedback attenuation factor provided by the feedback
network in useꢀ
SINGLE-SUPPLY VOLTAGE-TO-FREQUENCY
CONVERTER
The circuit shown in Figure 47 uses the AD822 to drive a low
power timer that produces a stable pulse of width t±ꢀ The positive
going output pulse is integrated by R± and C± and used as one
input to the AD822 that is connected as a differential integratorꢀ
The other input (nonloading) is the unknown voltage, VINꢀ The
AD822 output drives the timer trigger input, closing the overall
feedback loopꢀ
5
4
3
10V
U4
REF02
= 5V
C5
0.1µF
V
REF
2
4
6
5
CMOS
74HCO4
OUT2
OUT1
R
C3
0.1µF
3
SCALE
10kΩ
2
1
U3A
2
U3B
3
1
4
U2
CMOS 555
R2
499kΩ
1%
0.01µF, 2%
C1
300
1k
3k
10k
30k
8
4
R3
116kΩ
CAPACITIVE LOAD FOR 20° PHASE MARGIN (pF)
U1
R
V+
6
3
5
THR
+
OUT
CV
R1
1/2
2
7
499kΩ
1%
TR
AD822B
V
–
DIS
IN
GND
C2
0.01µF
2%
C
L
1
R
F
C4
0.01µF
0V TO 2.5V
FULL SCALE
R1
NOTES
1. f
= V
/
(V
× t ), t = 1.1 × R3 × C6.
1 1
Figure 45. Noise Gain vs. Capacitive Load Tolerance
OUT
IN
= 25kHzRfESFAS SHOWN.
2. R3 = 1% METAL FILM <50ppm/°C TC.
3. R = 10% 20T FILM <100ppm/°C TC.
SCALE
4. t = 33µF FOR f
= 20kHz @ V = 2.0V.
OUT IN
1
Figure 47. Single-Supply Voltage-to-Frequency Converter
Typical AD822 bias currents of 2 pA allow Mꢂ range source
impedances with negligible dc errorsꢀ Linearity errors on the
order of 0ꢀ0±% full scale can be achieved with this circuitꢀ This
performance is obtained with a 5 V single supply that delivers
less than ± mA to the entire circuitꢀ
Rev. I | Page 19 of 24
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