Datasheet

SCLK
DAC088S085
SYNC
V
OUTA
V
OUTB
V
OUTC
V
OUTD
V
REF1
D
IN
D
OUT
V
REF2
V
OUTE
V
OUTF
V
OUTG
V
OUTH
Set offset and gain
Programmable I
SOURCE
Set Limits for Range Detector
+V
- V
+
-
+
-
V
IN
Bipolar Output Swing
Control (Valve, Damper, Robotics,
Process Ctrl) or Voltage Setpoint
(Battery Ctrl, Signal Trigger)
Setting Sensor Drive or Supply
(Add buffer for sensor with low
input impedance)
V
REF
ADC121S625
Set ADC Reference
Sensor
Signal
(Ch A - Ch D)
(Ch E - Ch H)
3V or 5V Reference
3V or 5V Reference
Output to Another
DAC (Daisy Chain)
DAC088S085
SNAS424C –AUGUST 2007–REVISED MARCH 2013 www.ti.com
Figure 38. Industrial Application
ADC Reference
Figure 39 shows Channel A of the DAC088S085 providing the drive or supply voltage for a bridge sensor. By
having the sensor supply voltage adjustable, the output of the sensor can be optimized to the input level of the
ADC monitoring it. The output of the sensor is amplified by a fixed gain amplifier stage with a differential gain of 1
+ 2 × (R
F
/ R
I
). The advantage of this amplifier configuration is the high input impedance seen by the output of
the bridge sensor. The disadvantage is the poor common-mode rejection ratio (CMRR). The common-mode
voltage (V
CM
) of the bridge sensor is half of Channel A's DAC output. The V
CM
is amplified by a gain of 1V/V by
the amplifier stage and thus becomes the bias voltage for the input of the ADC121S705. Channel B of the
DAC088S085 is providing the reference voltage to the ADC121S705. The reference for the ADC121S705 may
be set to any voltage from 1V to 5V, providing the widest dynamic range possible.
The reference voltage for Channel A and B is powered by an external 5V power supply. Since the 5V supply is
common to the sensor supply voltage and the reference voltage of the ADC, fluctuations in the value of the 5V
supply will have a minimal effect on the digital output code of the ADC. This type of configuration is often referred
to as a "Ratio-metric" design. For example, an increase of 5% to the 5V supply will cause the sensor supply
voltage to increase by 5%. This causes the gain or sensitivity of the sensor to increase by 5%. The gain of the
amplifier stage is unaffected by the change in supply voltage. The ADC121S705 on the other hand, also
experiences a 5% increase to its reference voltage. This causes the size of the ADC's least significant bit (LSB)
to increase by 5%. As a result of the sensor's gain increasing by 5% and the LSB size of the ADC increasing by
the same 5%, there is no net effect on the circuit's performance. It is assumed that the amplifier gain is set low
enough to allow for a 5% increase in the sensor output. Otherwise, the increase in the sensor output level may
cause the output of the amplifiers to clip.
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