AD1671
Table III. Output Data Format
Input
Range
Analog
Inputl
Digital
Output
Coding
OTR2
0 V to +2.5 V
Straight Binary
≤
0 V
+2.5 V
≥ +2.5003 V
–0.0003 V
0000 0000 0000
0000 0000 0000
1111 1111 1111
1111 1111 1111
1
0
0
1
0 V to +5 V
Straight Binary
≤ –0.0006 V
0 V
+5 V
0000 0000 0000
0000 0000 0000
1111 1111 1111
1111 1111 1111
1
0
0
1
≥ +5.0006 V
–2.5 V to +2.5 V Offset Binary
≤ –2.5006 V
–2.5 V
+2.5 V
0000 0000 0000
0000 0000 0000
1111 1111 1111
1111 1111 1111
1
0
0
1
≥ +2.4994 V
–5 V to +5 V
Offset Binary
≤ –5.0012 V
–5 V
+5 V
0000 0000 0000
0000 0000 0000
1111 1111 1111
1111 1111 1111
1
0
0
1
≥ +4.9988 V
–2.5 V to +2.5 V Twos Complement ≤ –2.5006 V
1000 0000 0000
1000 0000 0000
0111 1111 1111
0111 1111 1111
1
0
0
1
(Using MSB)
–2.5 V
+2.5 V
≥ +2.4994 V
–5 V to +5 V
Twos Complement ≤ –5.0012 V
1000 0000 0000
1000 0000 0000
0111 1111 1111
0111 1111 1111
1
0
0
1
(Using MSB)
–5 V
+5 V
≥ +4.9988 V
NOTES
1Voltages listed are with offset and gain errors adjusted to zero.
2Typical performance.
connected to +5 V. It is possible to connect REF OUT to +5 V
due to its output circuit implementation which shuts down the
reference.
OUTPUT DATA FORMAT
The AD1671 provides both MSB and MSB outputs, delivering
data in positive true straight binary for unipolar input ranges
and positive true offset binary or twos complement for bipolar
input ranges. Straight binary coding is used for systems that ac-
cept positive-only signals. If straight binary coding is used with
bipolar input signals, a 0 V input would result in a binary output
of 2048. The application software would have to subtract 2048
to determine the true input voltage. Host registers typically per-
form math on signed integers and assume data is in that format.
Twos complement format minimizes software overhead which is
especially important in high speed data transfers, such as a
DMA operation. The CPU is not bogged down performing data
conversion steps, hence the total system throughput is increased.
ILOGIC VS. CONVERSION RATE
Figure 15 is the typical logic supply current vs. conversion rate
for various capacitor loads on the digital outputs.
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
CL = 50pF
OPTIONAL EXTERNAL REFERENCE
The AD1671 includes an onboard +2.5 V reference. The refer-
ence input pin (REF IN) can be connected to reference output
pin (REF OUT) or a standard external +2.5 V reference can be
selected to meet specific system requirements. Fast switching in-
put dependent currents are modulated at the reference input.
The reference input voltage can be held with the use of a capaci-
tor. To prevent the AD1671’s onboard reference from oscil-
lating when not connected to REF IN, REF OUT must be
CL = 30pF
CL = 0pF
1.0
0.5
1k
10k
100k
1M
CONVERSION RATE – Hz
Figure 15. ILOGIC vs. Conversion Rate for Various
Capacitive Loads on the Digital Outputs
REV. B
–11–
ADI [ ADI ]
