PGA GAIN
Input Select = 3H—Convert V1-3 via SH2, V2-3 via SH4,
and V3-3 (V1-3 and V2-3 are from the value sampled in a
preceding conversion cycle with Input Select = 4H, 5H or
6H).
The PGA gain is determined by the Gain Select portion (bits
8 and 9) in the SERIN word (see Figure 2). There is one gain
input that sets the same gain for all three PGAs. The gain
values and allowable full scale inputs are shown in Table II.
Input Select = 2H—Convert V1-3 via SH1, V2-3 via SH3,
and V3-3 (V1-3 is sampled on SH2 in this conversion cycle).
For channels one and two the PGAs set the gain for all three
analog inputs. For the third channel, only the V3-1 input is
gain changed by the PGA. Inputs V3-2, V3-3 and V3-4 are
connected to ADC3 at a fixed gain of 1V/V regardless of the
Gain Select value.
Input Select = 1H—Input V3-4 is converted by PGA3/
ADC3. The output of the asynchronous sample holds, SH6
and SH7, are converted by PGA1/ADC1 and PGA2/ADC2,
respectively. Note that the inputs to SH6 and SH7 are
determined by previous Input Select values (see Table IV).
Thus, to properly convert the output of one of the asynchro-
nous sample holds it is first necessary to choose its input
with a previous conversion cycle. Also, the output of SH6 or
SH7 will only be converted if ASH goes low before the
CONV command is received.
GAIN
SELECT0-1
GAIN
SETTING
FULL SCALE
INPUT
0H
1H
2H
3H
5.0V/V
2.5V/V
1.25V/V
1.0V/V
±0.5V
±1.0V
±2.0V
±2.5V
Input Select = 0H—V3-4 is converted by PGA3/ADC3. The
inputs to PGA1/ADC1 and PGA2/ADC2 are undefined.
TABLE II. Gain Select Information.
INPUT MULTIPLEXER AND
SAMPLE HOLD SELECTION
CONVERSIONS FROM THE
ASYNCHRONOUS SAMPLE HOLDS
The Input Select portion of the SERIN word (bits 10, 11 and
12) (see Figure 2) are decoded and determine the
open/closed condition of the multiplexer switches. This in
turn determines which input signals are connected to the
sample holds and which sample holds are connected to the
PGAs/ADCs.
Decoding the Input Select value also determines which
inputs are applied to the two asynchronously controlled
sample holds SH6 and SH7. (See Table IV.) One of the three
possible inputs is selected by the Input Select value being 4,
5, or 6.
The “No Effect” states indicate that these values of Input
Select have no effect on the multiplexers at the input of SH6
and SH7. When one of the “No Effect” values of Input Select
is presented, the multiplexers will not be changed (i.e., their
condition is determined by the last 4, 5, or 6 value of Input
Select that existed prior to the “No Effect” state).
INPUT SIGNALS FOR PGAs/ADCs
Table III shows the relationships between the value of Input
Select0-2 and the signals that are converted.
INPUT SELECT0-2
ANALOG SIGNAL CONNECTED TO
PGAX/ADCX
Note that Input Select = 1H presents the output of SH6 and
SH7 (1ASHX and 2ASHX) to PGA1/ADCl and PGA2/ADC2,
respectively (see Table III). Therefore, in order to properly
convert the asynchronous sampled signals, it is first neces-
sary to choose an input signal (Input Select equal 5 or 6 in
Table IV) with one load/convert cycle and then convert the
sample hold output (Input Select = 4 in Table III) in a
following conversion cycle.
HEX
BINARY
CODE
CODE
PGA1/ADC1
PGA2/ADC2
PGA3/ADC2
0H
1H
2H
3H
4H
5H
6H
7H
000
001
010
011
100
101
110
111
Undefined
V1-X via SH6
V1-3 via SH1
V1-3 via SH2
V1-2
Undefined
V3-4
V3-4
V3-3
V3-3
V3-2
V3-2
V3-2
V3-1
(1)
(1)
V2-X via SH7
V2-3 via SH3
V2-3 via SH4
V2-2
V1-2
V1-2
V1-1
V2-2
V2-2
V2-1
NOTE: (1) See Table IV for Operation.
INPUT SELECT0-2
ANALOG SIGNAL CONNECTED TO
HEX
CODE
BINARY
CODE
TABLE III. Input Controls for Synchronous Sample Holds.
SH6
SH7
0H
1H
2H
3H
4H
5H
6H
7H
000
001
010
011
100
101
110
111
No Effect
No Effect
No Effect
No Effect
Open
V1-3
V1-2
No Effect
No Effect
No Effect
No Effect
No Effect
Open
V2-3
V1-2
No Effect
Input Select = 7H—Synchronously sample and convert
input signals V1-1, V2-1, and V3-1
.
Input Select = 4H, 5H, 6H—Synchronously sample and
convert input signals V1-2, V2-2, and V3-2. These codes also
cause SH2 and SH4 to sample their inputs. Values 4H, 5H, 6H
have different effects on the inputs to SH6 and SH7 (see
Table IV).
TABLE IV. Input Controls for Asynchronous Sample Holds.
®
ADS7833
9
BB [ BURR-BROWN CORPORATION ]
