AD22050 Analog Devices, Inc., AD22050 Datasheet - Page 6

• Manufacturer Part Number: AD22050
• Description: Single-supply Sensor Interface Amplifier
• Manufacturer: Analog Devices, Inc.

AD22050
To produce a full-scale output of +4 V, a gain 40 is used, adjust-
able by 5% to absorb the tolerance in the sense resistor. There is
sufficient headroom to allow at least a 10% overrange (to +4.4 V).
The roughly triangular voltage across the sense resistor is aver-
aged by a single-pole low-pass filter, here set with a corner fre-
quency of f
at 100 Hz. A higher rate of attenuation can be obtained by a
two-pole filter having f
though this circuit uses two separate capacitors, the total capaci-
tance is less than half that needed for the single-pole filter.
STRAIN GAGE INTERFACE: MIDSCALE OFFSET
FEATURE
The AD22050 can be used to interface a strain gage to a subse-
quent process where only a single supply voltage is available. In
this application, the midscale offset feature is valuable, since the
output of the bridge may have either polarity. Figure 12 shows
typical connections.
Figure 12. Typical Connections for a Strain Gage Interface
Using the Offset Feature
The offset is obtained by connecting Pin 7 (OFS) to the supply
voltage. In this way, the output of the AD22050 is centered to
midway between the supply and ground. In many systems the
supply will also serve as the reference voltage for a subsequent
A/D converter. Alternatively, Pin 7 may be tied to the reference
voltage from an independent source. The AD22050 is trimmed
to guarantee an accuracy of 2% on the 0.5 ratio between the
voltage on Pin 7 and the output.
+V
CMOS DRIVER
CHASSIS
S
Figure 11. Illustration of Two-Pole Low-Pass Filtering
R
R
(BATTERY)
FLYBACK
V
G
DIODE
R
R
C
= 3.6 Hz, which provides about 30 dB of attenuation
100m
+IN OFS +V
–IN GND A1
AD22050
POWER
DARLINGTON
C
SOLENOID
LOAD
= 20 Hz, as shown in Figure 11. Al-
+IN
–IN GND A1
S
AD22050
OUT
A2
OFS +V
OPTIONAL
LP FILTER
100k
V
OS
(SETS GAIN
S
NULL
C
TO
OUT
A2
125k
100)
CORNER FREQUENCY
(0.05 F FOR f
127k
432k
50k
R
10k
ANALOG COMMON
= 1Hz- F
ANALOG OUTPUT
L
C
C
ANALOG
OUTPUT
ANALOG
COMMON
+5V
= 20Hz)
+V
S
–6–
An ac excitation of up to 2 V can also be used because the
common-mode range of the AD22050 extends to –1 V. Assum-
ing a full-scale bridge output (V
might be used to provide an output of 1 V (a full-scale range
of +1.5 V to +3.5 V). This gain is achieved using the method
discussed in connection with Figure 5. Note that the gain-
setting resistor does not affect the accuracy of the midscale
offset. (However, if the gain were lowered, using a resistor to
ground, this offset would no longer be accurate.) A V
pot is included for illustrative purposes. One-, two- and three-
pole filtering can also be implemented, as discussed in the
Low-Pass Filtering section.
Using the Midscale Offset Feature
Figure 13 shows a more detailed schematic of the output am-
plifier A2. Because this is a single supply device, the output
stage has no pull-down transistor. Such a transistor would limit
the minimum output to several hundred millivolts above
ground. When using the AD22050 in unipolar mode (Pin 7
grounded), the resistors making up the feedback network also
act as a pull-down for the output stage.
If the output is called upon to source current (not sink), then it
can swing almost completely to ground (within 20 mV). How-
ever, if the offset pin is connected to some positive voltage
source, this source will “pull up” the output voltage, thereby
limiting the minimum output swing. With no external load the
minimum output voltage possible is V
7 is connected to +5 V, the minimum output voltage is equal
to the offset voltage of 2.5 V. By adding an additional load, as
shown, the output swing toward ground can be extended.
The relationship is described by:
*This 20 k resistor is internal to the AD22050 and can vary by 30%.
where R
cannot be made arbitrarily small since this would require exces-
sive current from the output. The output current should be
limited to 5 mA total.
Figure 13. Detailed Schematic of Output Amplifier A2
L
A2
is an externally applied load resistor. However, R
V
OUT
+V
95k
S
1
2
V
OFS
10k
20k
G
) of 10 mV, a gain of 100
R
L
GND
OFS
20k
R
20 k *
L
/2. For example, if Pin
OFS
OUT
R
L
OS
nulling
REV. C
L