Datasheet

Data Sheet AD822
Rev. J | Page 19 of 24
90
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.... .... .... ....
....
.... .... ....
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.... .... .... ....
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100
0%
10
20mV 2µs
00874-044
Figure 44. Small Signal Response of AD822 as
Unity-Gain Follower Driving 350 pF
Figure 45 is a plot of noise gain vs. capacitive load that results in
a 20° phase margin for the AD822. Noise gain is the inverse of
the feedback attenuation factor provided by the feedback
network in use.
1
2
3
4
5
300 1k 3k 10k 30k
CAPACITIVE LOAD FOR 20° PHASE MARGIN (pF)
NOISE GAIN
1+
R
F
R
1
R1
R
F
C
L
00874-045
Figure 45. Noise Gain vs. Capacitive Load Tolerance
Figure 46 shows a method for extending capacitance load drive
capability for a unity-gain follower. With these component values,
the circuit drives 5000 pF with a 10% overshoot.
20pF
100Ω
20kΩ
8
4
V
IN
V
+
0.01µF
V
OUT
0.01µF
C
L
1/2
AD822
+
–
V–
00874-046
Figure 46. Extending Unity-Gain Follower Capacitive Load Capability
Beyond 350 pF
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
1
. The positive
going output pulse is integrated by R1 and C1 and used as one
input to the AD822 that is connected as a differential integrator.
The other input (nonloading) is the unknown voltage, V
IN
. The
AD822 output drives the timer trigger input, closing the overall
feedback loop.
2
3
4
6
5
U3A
U3B
C3
0.1
µF
123
4
OUT2
OUT1
U1
C1
U2
CMOS 555
THR
TR
DIS
GND
OUT
CV
1
2
3
4
5
6
7
8
RV+
C4
0.01µF
R3
116kΩ
0.01µF, 2%
CMOS
74HCO4
R
SCALE
10kΩ
U4
REF02
V
REF
= 5V
10V
C5
0.1µF
R2
499kΩ
1%
R1
499kΩ
1%
V
IN
C2
0.01µF
2%
0V TO 2.5V
FULL SCALE
1/2
AD822B
+
–
NOTES
1. f
OUT
= V
IN
/
(V
REF
×
t
1
), t
1
= 1.1 × R3 × C6.
2. R3 = 1% METAL FILM <50ppm/°C TC.
3. R
SCALE
= 10% 20T FILM <100ppm/°C TC.
4. t
1
= 33µF FOR f
OUT
= 20kHz @ V
IN
= 2.0V.
= 25kHz
f
S
AS SHOWN.
00874-047
Figure 47. Single-Supply Voltage to Frequency Converter
Typical AD822 bias currents of 2 pA allow MΩ range source
impedances with negligible dc errors. Linearity errors on the
order of 0.01% full scale can be achieved with this circuit. This
performance is obtained with a 5 V single supply that delivers
less than 1 mA to the entire circuit.