Datasheet AD22057 (Analog Devices) - 6

AD22057
To produce a full-scale output of +4 V, a gain ×40 is used, adjust- An ac excitation of up to ± 2 V can also be used because the able by ±5% to absorb the tolerance in the sense resistor. There is common-mode range of the AD22057 extends to –1 V. Assum- sufficient headroom to allow at least a 10% overrange (to +4.4 V). ing a full-scale bridge output (VG) of ± 10 mV, a gain of ×100 The roughly triangular voltage across the sense resistor is aver- might be used to provide an output of ± 1 V (a full-scale range aged by a single-pole low-pass filter, here set with a corner fre- of +1.5 V to +3.5 V). This gain is achieved using the method quency of fC = 3.6 Hz, which provides about 30 dB of attenuation discussed in connection with Figure 5. Note that the gain- at 100 Hz. A higher rate of attenuation can be obtained by a setting resistor does not affect the accuracy of the midscale two-pole filter having fC = 20 Hz, as shown in Figure 11. Al- offset. (However, if the gain were lowered, using a resistor to though this circuit uses two separate capacitors, the total capaci- ground, this offset would no longer be accurate.) A VOS nulling tance is less than half that needed for the single-pole filter. pot is included for illustrative purposes. One-, two- and three- pole filtering can also be implemented, as discussed in the
+5V
Low-Pass Filtering section.
+VS (BATTERY) SOLENOID LOAD ANALOG Using the Midscale Offset Feature OUTPUT
Figure 13 shows a more detailed schematic of the output am-
FLYBACK DIODE 432k
V plifier A2. Because this is a single supply device, the output
+IN OFS +VS OUT C
stage has no pull-down transistor. Such a transistor would limit
100m
V
AD22057 50k
V the minimum output to several hundred millivolts above
–IN GND A1 A2
ground. When using the AD22057 in unipolar mode (Pin 7
127k
V grounded), the resistors making up the feedback network also act as a pull-down for the output stage.
CMOS DRIVER CORNER FREQUENCY C = 1Hz-
m
F +VS (0.05
m
F FOR fC= 20Hz) A2 CHASSIS POWER ANALOG DARLINGTON COMMON OUT
Figure 11. Illustration of Two-Pole Low-Pass Filtering
10k
V
RL 95k
V
20k
V
OFS STRAIN GAGE INTERFACE: MIDSCALE OFFSET 20k
V
FEATURE
The AD22057 can be used to interface a strain gage to a subse-
GND
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 Figure 13. Detailed Schematic of Output Amplifier A2 typical connections. If the output is called upon to source current (not sink), then it can swing almost completely to ground (within 20 mV). How-
+VS
ever, if the offset pin is connected to some positive voltage source, this source will “pull up” the output voltage, thereby
ANALOG OUTPUT R
limiting the minimum output swing. With no external load the
R +IN OFS +V 125k
V
S OUT (SETS GAIN
minimum output voltage possible is V
V
OFS/2. For example, if Pin
G AD22057 TO
3
100)
7 is connected to +5 V, the minimum output voltage is equal
–IN GND A1 A2 R R RL
to the offset voltage of 2.5 V. By adding an additional load, as
10k
V
100k
V shown, the output swing toward ground can be extended.
VOS NULL
The relationship is described by:
OPTIONAL LP FILTER ANALOG COMMON
R V > 1 L OUT V 2 OFS R +20 kΩ* Figure 12. Typical Connections for a Strain Gage Interface L Using the Offset Feature *This 20 kΩ resistor is internal to the AD22057 and can vary by ± 30%. where R The offset is obtained by connecting Pin 7 (OFS) to the supply L is an externally applied load resistor. However, RL cannot be made arbitrarily small since this would require exces- voltage. In this way, the output of the AD22057 is centered to sive current from the output. The output current should be midway between the supply and ground. In many systems the limited to 5 mA total. 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 AD22057 is trimmed to guarantee an accuracy of ± 2% on the 0.5 ratio between the voltage on Pin 7 and the output. –6– REV. B