FUNCTIONAL DESCRIPTION
ships). The ADCs are preceded by programmable gain
amplifiers. (Table II gives gain select information). For
channels one and two, the PGAs are effective 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.
(See Figure 1)
ADCs AND PGAs
The ADS7833 contains three signal channels each with a
12-bit analog-to-digital converter output. The ADCs operate
synchronously and their serial outputs occur simultaneously.
(Table VI gives the analog input/digital output relation-
2
V
V
V
1-1
1-2
1-3
2
2
2
2
SH
SH
1
2
2
2
2
2
S
ADC
PGA
2
OUT1
1
1
2
Ref
Conv
SH
6
ASH
2
2
2
V
V
V
2-1
2-2
2-3
2
2
SH
SH
3
2
2
2
2
2
PGA
ADC
S
OUT2
4
2
2
2
Ref
Conv
SH
7
V
Through V
3-2
3-4
ASH
2
2
2
2
V
V
V
V
3-1
3-2
3-3
3-4
2
Only
2
2
SH
PGA
3
ADC
S
OUT
5
3
3
V
3-1
Input Gain
Select Select
Ref
Ref Conv
2.5V
Ref
Decoder
CAP
REF
3
2
Conv
Sample
IN
Input Setup
Register
SER
IN
DAC
Input
Control
Logic
8
DAC
8-Bit
A
OUT
ASH
CLK
CONV
BUSY
DCLOCK
FIGURE 1. Functional Diagram.
SAMPLE HOLDS
the quadrature inputs of a given position sensor at the same
time (even though they are converted on successive conver-
sion cycles) (see Table III), so that their values are captured
at the same shaft position.
The ADS7833 contains seven sample holds. Five of them
(SH1 through SH5) sample simultaneously and have their
sample/hold timing internally synchronized. (The timing is
shown in Figure 2).
The ADS7833 also has the capability for limited asynchro-
nous sampling. The sampling of SH6 and SH7 is controlled
asynchronously by the control signal ASH (see Table III).
This allows two inputs each on channel 1 and channel 2 (see
Table IV) to be sampled asynchronously from the timing of
the other sample holds. This can be useful in motor control
applications where the two inputs for each channel come
from a position sensor and it is desired to sample based on
position sensor timing rather than system clock timing.
Three of the sample holds (SH1, SH3, and SH5) are con-
nected to the input multiplexers so that they can provide
simultaneous sampling for all of their channels inputs. In
addition, SH2 and SH4 simultaneously sample the third input
of their channels (Vl–3 and V2–3, respectively). This is useful
in motor control applications where V1-2 and Vl-3 are the
quadrature inputs for one position sensor, and V2-2 and
V2-3 are the quadrature inputs for a second position sensor
(see Figure 6). In that application, it is desirable to sample
®
ADS7833
7
BB [ BURR-BROWN CORPORATION ]
