AD8290 Datasheet, PDF (16/20 Pages) Analog Devices – G = 50, CMOS Sensor Amplifier with Current Excitation

AD8290
APPLICATIONS INFORMATION
TYPICAL CONNECTIONS
Figure 45 shows the typical connections for single-supply
operation when used with a sensor bridge.
CURRENT EXCITATION
In Figure 45, RSET is used to set the excitation current sourced at
the IOUT pin. The formula for the excitation current IOUT is
IOUT = (900/RSET) mA
where RSET is the resistor between Pin 10 (GND) and Pin 11
(RSET).
The AD8290 is internally set by the factory to provide the
current excitation described by the previous formula (within the
tolerance range listed in Table 1). The range of RSET is 692 Ω to
3 kΩ, resulting in a corresponding IOUT of 1300 μA to 300 μA,
respectively.
ENABLE/DISABLE FUNCTION
Pin 3 (ENBL) provides the enabling/disabling function of the
AD8290 to conserve power when the device is not needed. A
Logic 1 turns the part on and allows it to operate normally. A
Logic 0 disables the output and excitation current and reduces
the quiescent current to less than 10 μA.
The turn-on time upon switching Pin 3 high is dominated
by the output filters. When the device is disabled, the output
becomes high impedance, enabling the muxing application of
multiple AD8290 instrumentation amplifiers.
OUTPUT FILTERING
Filter Capacitor CFILTER is required to limit the amount of
switching noise present at the output. The recommended
bandwidth of the filter created by CFILTER and an internal
100 kΩ is 235 Hz. Select CFILTER based on
CFILTER = 1/(235 × 2 × π × 100 kΩ) = 6.8 nF
For bandwidths greater than 10 Hz, an additional single-pole
RC filter of 235 Hz is required on the output, which is also
recommended when driving an ADC requiring an antialiasing
filter. Internal to the AD8290 is a series 10 kΩ resistor at the
output (R3 in Figure 43) and using an external 68 nF capacitor
to ground produces an RC filter of 235 Hz on the output as well.
These two filters produce an overall bandwidth of approximately
160 Hz for the output signal.
In addition, when driving low impedances, the internal series
10 kΩ resistor creates a voltage divider at the output. If it is
necessary to access the output of the internal amplifier prior
to the 10 kΩ resistor, it is available at the CF2 pin.
For applications with low bandwidths (<10 Hz), only the first
filter capacitor (CFILTER) is required. In this case, the high
frequency noise from the auto-zero amplifier (output amplifier)
is not filtered before the following stage.
CLOCK FEEDTHROUGH
The AD8290 uses two synchronized clocks to perform
autocorrection. The input voltage-to-current amplifiers
are corrected at 60 kHz.
Trace amounts of these clock frequencies can be observed at
the output. The amount of feedthrough is dependent upon the
gain because the autocorrection noise has an input- and output-
referred term. The correction feedthrough is also dependent
upon the values of the external capacitors, C2 and CFILTER.
R_SET
692Ω TO 3kΩ
C_FILTER
6.8nF
11
RSET
6
CF1
5
CF2
13 IOUT
14 VINN
AD8290
3
ENBL
VCC 2
GND 10
5.0V
C1
0.1µF
C_BRIDGE
15 VINP
VOUT 4
NC NC NC NC NC NC
1
7
8
9
12
16
V_OUT
C2
68nF
NC = NO CONNECT
NOTES
LAYOUT CONSIDERATIONS:
1. KEEP C1 CLOSE TO PIN 2 AND PIN 10.
2. KEEP R_SET CLOSE TO PIN 11.
Figure 45. Typical Single-Supply Connections
Rev. B | Page 16 of 20