AD822
20mV
100
90
2µs
.... .... .... .... .... .... .... .... .... ....
capability for a unity-gain follower. With these component
values, the circuit drives 5000 pF with a 10% overshoot.
V+
8
V
IN
0.01µF
+
–
1/2
0.01µF
100Ω
AD822
4
C
L
V
OUT
10
0%
.... .... .... .... .... .... .... .... .... ....
00874-044
V–
20pF
00874-046
20kΩ
Figure 44. Small Signal Response of AD822 as
Unity-Gain Follower Driving 350 pF
Figure 46. Extending Unity-Gain Follower Capacitive Load Capability
Beyond 350 pF
a 20° phase margin for the AD822. Noise gain is the inverse of
the feedback attenuation factor provided by the feedback
network in use.
5
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.
10V
U4
REF02
V
REF
= 5V
R
SCALE
10kΩ
CMOS
74HCO4
U3B
3
4
4
R
F
R
1
NOISE GAIN 1+
3
C5
0.1µF
2
6
5
2
3
4
U3A
2
C3
0.1µF
1
U2
CMOS 555
4
6
2
7
R
THR
TR
DIS
GND
1
CV
8
V+
OUT
3
5
OUT2
OUT1
1
300
1k
3k
10k
CAPACITIVE LOAD FOR 20° PHASE MARGIN (pF)
30k
R2
499kΩ
1%
U1
R1
499kΩ
1%
0.01µF, 2%
C1
R3
116kΩ
+ 1/2
AD822B
–
V
IN
R
F
R1
C
L
00874-045
C2
0.01µF
2%
0V TO 2.5V
FULL SCALE
C4
0.01µF
Figure 45. Noise Gain vs. Capacitive Load Tolerance
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.
Rev. I | Page 19 of 24
00874-047
NOTES
1. f
OUT
= V
IN/
(V
REF ×
t
1
), t
1
= 1.1
×
R3
×
C6.
= 25kHz
f
S
AS SHOWN.
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.
AD [ ANALOG DEVICES ]
