AD790 Analog Devices, Inc., AD790 Datasheet - Page 7

• Manufacturer Part Number: AD790
• Description: Fast, Precision Comparator
• Manufacturer: Analog Devices, Inc.

REV. D
Window Comparator for Overvoltage Detection
The wide differential input range of the AD790 makes it suitable
for monitoring large amplitude signals. The simple overvoltage
detection circuit shown in Figure 8 illustrates direct connection
of the input signal to the high impedance inputs of the comparator
without the need for special clamp diodes to limit the differen-
tial
input voltage across the inputs.
Single Supply Ground Referred Overload Detector
The AD790 is useful as an overload detector for sensitive loads
that must be powered from a single supply. A simple ground
referenced overload detector is shown in Figure 8. The com-
parator senses a voltage across a PC board trace and compares
that to a reference (trip) voltage established by the comparator’s
minus supply current through a 2.7 Ω resistor. This sets up a
10 mV reference level that is compared to the sense voltage.
V
IN
0.1µF
+IN
–IN
Figure 7. Typical Power Supply Connections
(N, Q Package Pinout)
Figure 8. Overvoltage Detector
(N, Q Package Pinout)
+V
–7.5V
+7.5V
S
2
3
V
0.1µF
= +12V, –V
LOGIC
+ 12V
AD790
0.1µF
1
4
= +5V
+
5V
8
3
2
S
3
2
+IN
–IN
= 0V
6
+15V
+15V
+V
AD790
AD790
5
1
–15V
1
–15V
S
510Ω
4
= +5V, –V
4
7
2
3
0.1µF
AD790
+5V
+5V
1
8
–
8
5V
4
OUT
S
+
= –5V, V
5V
8
6
6
0.1µF
5
5
6
0.1µF
0.1µF
5
0.1µF
510Ω
510Ω
LOGIC
0.1µF
7
7
7
+IN
–IN
= +5V
0.1µF
OUT
2
3
AD790
1
–
+V
15V
4
S
= +5V, –V
+
V
8
5V
LOGIC
OVERRANGE = 1
SIGN 1 = HIGH
6
= +5V
5
0.1µF
S
7432
= –15V
7
0 = LOW
0.1µF
OUT
–7–
–7–
The minus supply current is proportional to absolute tempera-
ture and compensates for the change in the sense resistance
with temperature. The width and length of the PC board trace
determine the resistance of the trace and consequently the trip
current level.
I
R
rho = resistance of a unit square of trace
Precision Full-Wave Rectifier
The high speed and precision of the AD790 make it suitable
for use in the wide dynamic range full-wave rectifier shown in
Figure 10. This circuit is capable of rectifying low level signals
as small as a few mV or as high as 10 V. Input resolution, propaga-
tion delay and op amp settling will ultimately limit the maximum
input frequency for a given accuracy level. Total comparator
plus switch delay is approximately 100 ns, which limits the
maximum input frequency to 1 MHz for clean rectification.
0.1µF
V
LIMIT
IN
SENSE
Figure 9. Ground Referred Overload Detector Circuit
(N, Q Package Pinout)
R
PC BOARD
≈ 10mV/100mA
SENSE
TRACE
= 10 mV/R
Figure 10. Precision Full-Wave Rectifier
(N, Q Package Pinout)
= rho (trace length/trace width)
3
2
+15V
AD790
1
–15V
4
+V
+5V
8
L
O
A
D
S
0.1µF
6
SENSE
0.1µF
5
510
10k
20k
7
Ω
Ω
Ω
2.7
2
3
Ω
Applying the
+
AD790
FET SWITCHES THE GAIN
FROM +1 TO –1
NMOS
FET
(R
1
5V
ON
4
2
3
10k
< 20 )
8
AD711
Ω
Ω
+15V
–15V
6
7
4
5
510Ω
0.1µF
0.1µF
0.1µF
7
6
AD790
OUTPUT
V
OUT