Datasheet

+
-
20 k:
20 k:
V
BIAS
LMP7731
+5V
DAC108S085
Controller
REF
V
A
+5V
4.7 PF
+
-
+
-
ADC121S705
REF
Controller
LMP7702
R
I
R
F
R
F
Bridge
Sensor
+5V
CSB
SCLK
DOUT
SYNCB
SCLK
DIN
REF
REF
DAC108S085
+5V
Channel B
Channel A
Av = 1 + 2
R
F
R
I
DAC108S085
SNAS423B –AUGUST 2007–REVISED MARCH 2013 www.ti.com
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.
Figure 39. Driving an ADC Reference
Programmable Attenuator
shows one of the channels of the DAC108S085 being used as a single-quadrant multiplier. In this configuration,
an AC or DC signal can be driven into one of the reference pins. The SPI interface of the DAC can be used to
digitally attenuate the signal to any level from 0dB (full scale) to 0V. This is accomplished without adding any
noticeable level of noise to the signal. An amplifier stage is shown in as a reference for applications where the
input signal requires amplification. Note how the AC signal in this application is ac-coupled to the amplifier before
being amplified. A separate bias voltage is used to set the common-mode voltage for the DAC108S085's
reference input to V
A
/ 2, allowing the largest possible input swing. The multiplying bandwidth of V
REF1,2
is
360kHz with a V
CM
of 2.5V and a peak-to-peak signal swing of 2V.
Figure 40. Programmable Attenuator
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