MAX1270 Maxim, MAX1270 Datasheet - Page 14

• Manufacturer Part Number: MAX1270
• Description: Multirange / +5V / 8-Channel / Serial 12-Bit ADCs
• Manufacturer: Maxim

The internally trimmed 2.50V reference is amplified
through the REFADJ buffer to provide 4.096V at REF.
Bypass REF with a 4.7µF capacitor to AGND and
REFADJ with a 0.01µF capacitor to AGND (Figure 13a).
The internal reference voltage is adjustable to ±1.5%
(±65 LSBs) with the reference-adjust circuit of Figure 1.
To use the REF input directly, disable the internal buffer
by tying REFADJ to V
REFADJ input eliminates the need to buffer the refer-
ence externally. When a reference is applied at
REFADJ, bypass REFADJ with a 0.01µF capacitor to
AGND. Note that when an external reference is applied
at REFADJ, the voltage at REF is given by:
Multirange, +5V, 8-Channel,
Serial 12-Bit ADCs
Figure 11. Internal Clock Mode, 13 Clocks/Conversion Timing
Figure 12. Internal Clock Mode, 16 Clocks/Conversion Timing
7-182
DOUT
A/D STATE
SSTRB
A/D STATE
SCLK
DIN
CS
SSTRB
DOUT
SCLK
___________________________________________________________________________________
DIN
CS
HIGH-Z
HIGH-Z
START
1
START SEL2 SEL1 SEL0
SEL2 SEL1 SEL0
1
IDLE
CONTROL BYTE Ø
CONTROL BYTE Ø
RNG
DD
RNG
ACQUISITION CONVERSION
BIP
PD1 PD0
ACQUISITION CONVERSION
BIP
(Figure 13b). Using the
PD1 PD0
16 SCLK
8
13 SCLK
HIGH-Z
8
External Reference
Internal Reference
D11 D10 D9
9
D11 D10 D9
9
D8
D8
RESULT Ø
D7
D7
D6
RESULT Ø
START SEL2 SEL1 SEL0
D6 D5 D4
14
D5 D4
13 SCLK
CONTROL BYTE 1
16
START SEL2 SEL1 SEL0
16
D3
17
D3
16 SCLK
(Figure 13c). At REF and REFADJ, the input impedance
is a minimum of 10kΩ for DC currents. During conver-
sions, an external reference at REF must be able to
deliver 400µA DC load currents and must have an out-
put impedance of 10Ω or less. If the reference has
higher output impedance or is noisy, bypass REF with a
4.7µF capacitor to AGND as close to the chip as possi-
ble.
With an external reference voltage of less than 4.096V
at REF or less than 2.5V at REFADJ, the increase in the
ratio of RMS noise to the LSB value (full-scale / 4096)
results in performance degradation (loss of effective
bits).
CONTROL BYTE 1
D2
RNG
D2
D1 D0
ACQUISITION CONVERSION
BIP
D1 D0
V
PD1 PD0
REF
RNG
ACQUISITION CONVERSION
BIP
= 1.6384 · V
PD1 PD0
24
HIGH-Z
D11
22
D10
REFADJ
D11
25
D9
24
D10
D8
RESULT 1
D9
D7
START
D8
CONTROL BYTE 2
(2.4 < V
D6
RESULT 1
D7
SEL2
D5
D6
SEL1
D4
D5
REF
SEL0
D3
D4
32
< 4.18)
CB 2
START
D3
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