AD7891
CIRCUIT DESCRIPTION
Reference
The AD7891 contains a single reference pin labelled REF OUT/
REF IN, which either provides access to the part’s own +2.5 V
internal reference or to which an external +2.5 V reference can
be connected to provide the reference source for the part. The
part is specified with a +2.5 V reference voltage. Errors in the
reference source will result in gain errors in the transfer function
of the AD7891 and will add to the specified full scale errors on
the part. They will also result in an offset error injected into the
attenuator stage.
The AD7891 contains an on-chip +2.5 V reference. To use this
reference as a reference source for the AD7891, simply connect
a 0.1
µF
disc ceramic capacitor from the REF OUT/REF IN pin
to REFGND. REFGND should be connected to AGND or the
analog ground plane. The voltage that appears at the REF OUT/
REF IN pin is internally buffered before being applied to the
ADC. If this reference is required for use external to the AD7891,
it should be buffered as the part has a FET switch in series with
the reference, resulting in a source impedance for this output of
2 kΩ nominal. The tolerance of the internal reference is
±
10 mV
at +25°C with a typical temperature coefficient of 25 ppm/°C
and a maximum error over temperature of
±
20 mV.
If the application requires a reference with a tighter tolerance or
if the AD7891 needs to be used with a system reference, then an
external reference can be connected to the REF OUT/REF IN
pin. The external reference will overdrive the internal reference
and thus provide the reference source for the ADC. The refer-
ence input is buffered before being applied to the ADC and the
maximum input current is
±
100
µA.
Suitable reference for the
AD7891 include the AD580, the AD680, the AD780 and the
REF43 precision +2.5 V references.
Analog Input Section
The input resistance for the
±5
V range is typically 20 kΩ. For
the
±
10 V input range the input resistance is typically 34.3 kΩ.
The resistor input stage is followed by the multiplexer and this
is followed by the high input impedance stage of the track/hold
amplifier.
The designed code transitions take place midway between suc-
cessive integer LSB values (i.e., 1/2 LSB, 3/2 LSBs, 5/2 LSBs,
etc.). LSB size is given by the formula, 1 LSB = FS/4096. Thus
for the
±
5 V range, 1 LSB = 10 V/4096 = 2.44 mV. For the
±
10 V range, 1 LSB = 20 V/4096 = 4.88 mV. Output coding is
determined by the FORMAT bit of the control register. The
ideal input/output code transitions are shown in Table I.
AD7891-2
Figure 6 shows the analog input section of the AD7891-2. Each
input can be configured for input ranges of 0 V to +5 V, 0 V to
+2.5 V or
±
2.5 V. For the 0 V to +5 V input range, the V
INXB
input is tied to AGND and the input voltage is applied to the
V
INXA
input. For the 0 V to +2.5 V input range, the V
INXA
and
V
INXB
inputs are tied together and the input voltage is applied to
both. For the
±
2.5 V input range, the V
INXB
input is tied to
+2.5 V and the input voltage is applied to the V
INXA
input. The
+2.5 V source must have a low output impedance. If the inter-
nal reference on the AD7891 is used, then it must be buffered
before being applied to V
INXB
. The V
INXA
and V
INXB
inputs are
symmetrical and fully interchangeable. Thus for ease of PCB
layout on the 0 V to +5 V range or the
±
2.5 V range, the input
voltage may be applied to the V
INXB
input while the V
INXA
input
is tied to AGND or +2.5 V.
REF OUT/REF IN
TO ADC
REFERENCE
CIRCUITRY
1.8k
V
INXA
1.8k
V
INXB
TO
MULTIPLEXER
2k
The AD7891 is offered as two part types, the AD7891-1 where
each input can be configured to have a
±
10 V or a
±5
V input
range and the AD7891-2 where each input can be configured to
have a 0 V to 2.5 V, 0 V to 5 V and
±
2.5 V input range.
AD7891-1
Figure 5 shows the analog input section of the AD7891-1. Each
input can be configured for
±
5 V or
±
10 V operation. For +5 V
operation, the V
INXA
and V
INXB
inputs are tied together and the
input voltage is applied to both. For
±10
V operation, the V
INXB
input is tied to AGND and the input voltage is applied to the
V
INXA
input. The V
INXA
and V
INXB
inputs are symmetrical and
fully interchangeable. Thus for ease of PCB layout on the
±10
V
range, the input voltage may be applied to the V
INXB
input while
the V
INXA
input is tied to AGND.
REF OUT/REF IN
TO ADC
REFERENCE CIRCUITRY
30k
V
INXA
2k
30k
V
INXB
15k
AGND
TO
MULTIPLEXER
7.5k
2.5V
REFERENCE
AD7891-2
AGND
Figure 6. AD7891-2 Analog Input Structure
The input resistance for both the 0 V to +5 V and
±
2.5 V ranges
is typically 3.6 kΩ. When an input is configured for 0 V to 2.5 V
operation, the input is fed into the high impedance stage of the
track/hold amplifier via the multiplexer and the two 1.8 kΩ
resistors in parallel.
The designed code transitions occur midway between successive
integer LSB values (i.e., 1/2 LSB, 3/2 LSBs, 5/2 LSBs etc.).
LSB size is given by the formula 1 LSB = FS/4096. Thus for the
0 V to +5 V range, 1 LSB = 5 V/4096 = 1.22 mV, for the 0 V
to +2.5 V range, 1 LSB = 2.5 V/4096 = 0.61 mV and for the
±
2.5 V range, 1 LSB = 5 V/4096 = 1.22 mV. Output coding is
determined by the FORMAT bit in the control register. The
ideal input/output code transitions for the
±2.5
V range are
shown in Table I. The ideal input/output code transitions for
the 0 V to +5 V range and the 0 V to +2.5 V range are shown in
Table II.
AD7891-1
2.5V
REFERENCE
Figure 5. AD7891-1 Analog Input Structure
–12–
REV. A
AD [ ANALOG DEVICES ]
