AD871
REV. A
–12–
REF IN
REF GND
AD871
+5V REF
2k
V
3.9k
V
R
R
T
2.5V
5k
V
Figure 21. Optional +5 V Reference Input Circuit
REFERENCE GROUND
The REF GND pin provides the reference point for both the
reference input and the reference output. When the internal ref-
erence is operating, it will draw approximately 500
μ
A of current
through the reference ground, so a low impedance path to the
external common is desirable. The AD871 can tolerate a fairly
large difference between REF GND and AGND, up to
±
1 V, without any performance degradation.
REFERENCE OUTPUT
The AD871 features an onboard, curvature compensated band-
gap reference that has been laser trimmed for both absolute
value and temperature drift. The output stage of the reference
was designed to allow the use of an external capacitor to limit
the wideband noise. As Figure 22 illustrates, a 1
μ
F capacitor on
the reference output is required for stability of the reference out-
put buffer
.
Note: If used, an external reference may become un-
stable with this capacitor in place.
REF IN
REF GND
AD871
0.1
m
F
REF OUT
1
m
F
+
Figure 22. Typical Reference Decoupling Connection
With this capacitor in place, the noise on the reference output is
approximately 28
μ
V rms at room temperature. Figure 23 shows
the typical temperature drift performance of the reference, while
Figure 24 illustrates the variation in reference voltage with load
currents.
The output stage is designed to provide at least 2 mA of output
current, allowing a single reference to drive up to four AD871s
or other external loads. The power supply rejection of the refer-
ence is better than 54 dB at dc.
2.55
2.45
125
2.48
2.46
–35
2.47
–55
2.51
2.49
2.50
2.52
2.53
2.54
105
85
65
45
25
5
–15
TEMPERATURE –
8
C
R
Figure 23. Reference Output Voltage vs. Temperature
2.50
2.40
1M
2.46
2.42
10k
2.44
1k
2.48
100k
R
REFERENCE OUTPUT LOAD –
V
Figure 24. Reference Output Voltage vs. Output Load
DIGITAL OUTPUTS
In 28-lead packages, the AD871 output data is presented in
twos complement format. Table III indicates offset binary and
twos complement output for various analog inputs.
Table III. Output Data Format
Analog Input
V
INA
–V
INB
Digital Output
Offset Binary
Twos Complement OTR
≥
0.999756 V
0.999268 V
0 V
–1 V
–1.000244 V
1111 1111 1111
1111 1111 1111
1000 0000 0000
0000 0000 0000
0000 0000 0000
0111 1111 1111
0111 1111 1111
0000 0000 0000
1000 0000 0000
1000 0000 0000
1
0
0
0
1
Users requiring offset binary encoding may simply invert the
MSB
pin. In the 44-terminal surface mount packages, both
MSB and
MSB
bits are provided.
The AD871 features a digital out-of-range (OTR) bit that goes
high when the input exceeds positive full scale or falls below
negative full scale. As Table III indicates, the output bits will be
set appropriately according to whether it is an out-of-range high
condition or an out-of-range low condition. Note that if the in-
put is driven beyond +1.5 V, the digital outputs may not stay at
+FS, but may actually fold back to midscale.