Datasheet

Data Sheet ADAS3022
Rev. B | Page 25 of 40
ANALOG INPUTS
Input Structure
The ADAS3022 uses a differential input structure between
IN[7:0] and COM or between IN[7:0]+ and IN[7:0]− of a
channel pair. The COM input is sampled identically such that
the same voltages can be present on inputs IN[7:0]. Therefore,
the selection of paired channels or all channels referenced to
one common point is available. Because all inputs are sampled
differentially, the ADAS3022 offers true high common-mode
rejection, whereas a discrete system would require the use of
additional instrumentation or a difference amplifier.
Figure 55 shows an equivalent circuit of the analog inputs. The
internal diodes provide ESD protection for the analog inputs
(IN[7:0] and COM) from the high voltage supplies (VDDH and
VSSH). Care must be taken to ensure that the analog input
signal does not exceed the supply rails by more than 0.3 V
because this can cause the diodes to become forward-biased and
to start conducting current. Note that if the auxiliary input pair
(AUX±) is used, the diodes provide ESD protection from only the
lower voltage AVDD (5 V) supply and the system analog ground
because these inputs are connected directly to the internal SAR
ADC circuitry.
C
PIN
C
PIN
IN[7:0]
OR COM
VSSH
AUX+
O
R AUX–
AGND
V
DDH
AVDD
MUX PGIA
10516-008
Figure 55. Equivalent Analog Input Circuit
Voltages beyond the absolute maximum ratings may cause
permanent damage to the ADAS3022 (see Table 4).
Programmable Gain
The ADAS3022 incorporates a programmable gain instru-
mentation amplifier with seven selectable ranges (±0.64 V,
±1.28 V, ±2.56 V, ±5.12 V, ±10.24 V, ±20.48 V, and ±24.576 V),
enabling the use of almost any direct sensor interface. The PGIA
settings are specified in terms of the maximum absolute differential
input voltage across a pair of inputs (for example, INx+ to INx−
or INx+ to COM). The power-on and default conditions are
preset to the ±20.48 V (PGIA = 111) input range.
Note that because the ADAS3022 can use any input type, such
as bipolar differential (antiphase or nonantiphase), bipolar single
ended, or pseudo bipolar, setting the PGIA is important to
make full use of the allowable input span.
Table 7 describes each differential input range and the
corresponding LSB size, PGIA bits settings, and PGIA gain.
Table 7. Differential Input Ranges, LSB Size, and PGIA
Settings
Differential Input Ranges,
INx+ − INx− (V)
LSB (μV) PGIA Bits
PGIA Gain
(V/V)
±24.576 781.25 000 0.16
±20.48 625 111 0.2
±10.24 312.5 001 0.4
±5.12 156.3 010 0.8
±2.56 78.13 011 1.6
±1.28 39.06 100 3.2
±0.64 19.53 101 6.4
Common-Mode Operating Range
The differential input common-mode voltage (V
CM
) range
changes according to the maximum input range selected and
the high voltage power supplies (VDDH and VSSH). Note that
the specified operating input voltage of any input pin (see the
Specifications section) requires 2.5 V of headroom from the
VDDH and VSSH supplies; therefore,
(VSSH + 2.5 V) ≤ INx/COM ≤ (VDDH − 2.5 V)
This section provides some examples of setting the PGIA for
various input signals. Note that the ADAS3022 always calculates
the difference between the IN+ and IN− signals.
Fully Differential, Antiphase Signals with a
Zero Common Mode
For a pair of 20.48 V p-p differential antiphase signals with a
zero common mode, the maximum differential voltage across
the inputs is 20.48 V, and the PGIA gain configuration should
be set to 111.
ADAS3022
INx+
INx+
+10.24
V
20.48V p-p
20.48V p-p
–
10.24
V
INx–
INx–
10516-009
Figure 56. Differential, Antiphase Inputs with a Zero Common Mode