MAX1295 Datasheet, PDF (15/20 Pages) Maxim Integrated Products – 265ksps, +3V, 6-/2-Channel, 12-Bit ADCs with +2.5V Reference and Parallel Interface

265ksps, +3V, 6-/2-Channel, 12-Bit ADCs
with +2.5V Reference and Parallel Interface
Table 5. Full-Scale and Zero-Scale for Unipolar and Bipolar Operation
UNIPOLAR MODE
Full Scale
Zero Scale
—
V_REF + COM
COM
—
BIPOLAR MODE
Positive Full Scale
Zero Scale
Negative Full Scale
V_REF/2 + COM
COM
-V_REF/2 + COM
OUTPUT CODE
111 . . . 111
111 . . . 110
FS = REF + COM
ZS = COM
100 . . . 010
100 . . . 001
100 . . . 000
011 . . . 111
011 . . . 110
011 . . . 101
1LSB = ^REF
4096
FULL-SCALE
TRANSITION
000 . . . 001
000 . . . 000
0 12
(COM)
2048
INPUT VOLTAGE (LSB)
FS
FS - 3/2LSB
Figure 8. Unipolar Transfer Function
Shutdown Mode
Shutdown mode turns off all chip functions that draw qui-
escent current, reducing the typical supply current to
2µA immediately after the current conversion is complet-
ed. A rising edge on WR causes the MAX1295/MAX1297
to exit shutdown mode and return to normal operation.
To achieve full 12-bit accuracy with a 4.7µF reference
bypass capacitor, 50µs is required after power-up.
Waiting this 50µs in standby mode instead of in full-
power mode can reduce power consumption by a factor
of 3 or more. When using an external reference, only
50µs is required after power-up. Enter standby mode by
performing a dummy conversion with the control byte
specifying standby mode.
Note: Bypass capacitors larger than 4.7µF between
REF and GND will result in longer power-up delays.
OUTPUT CODE
011 . . . 111
011 . . . 110
000 . . . 010
000 . . . 001
000 . . . 000
111 . . . 111
111 . . . 110
111 . . . 101
FS = ^REF + COM
2
ZS = COM
-FS = ^-REF + COM
2
1LSB
=
REF
4096
100 . . . 001
100 . . . 000
- FS
*COM V_REF/2
COM*
INPUT VOLTAGE (LSB)
Figure 9. Bipolar Transfer Function
+FS - 1LSB
Transfer Function
Table 5 shows the full-scale voltage ranges for unipolar
and bipolar modes. Figure 8 depicts the nominal unipo-
lar input/output (I/O) transfer function, and Figure 9
shows the bipolar I/O transfer function. Code transitions
occur halfway between successive-integer LSB values.
Output coding is binary, with 1LSB = (V_REF/4096).
Maximum Sampling Rate/
Achieving 300ksps
When running at the maximum clock frequency of
4.8MHz, the specified throughput of 265ksps is achieved
by completing a conversion every 18 clock cycles: 1
write cycle, 3 acquisition cycles, 13 conversion cycles,
and 1 read cycle. This assumes that the results of the
last conversion are read before the next control byte is
written. It is possible to achieve higher throughputs, up
to 300ksps, by first writing a control byte to begin the
______________________________________________________________________________________ 15