MAX1277 Datasheet, PDF (12/18 Pages) Maxim Integrated Products – 1.5Msps, Single-Supply, Low-Power, True-Differential, 12-Bit ADCs with Internal Reference

1.5Msps, Single-Supply, Low-Power, True-
Differential, 12-Bit ADCs with Internal Reference
OUTPUT CODE
111...111
111...110
111...101
FULL-SCALE
TRANSITION
000...011
000...010
000...001
000...000
012 3
FS = V_REF
ZS = 0
1 LSB = ^VREF
4096
DIFFERENTIAL INPUT
VOLTAGE (LSB)
FS
FS - 3/2 LSB
Figure 8. Unipolar Transfer Function (MAX1277 Only)
OUTPUT CODE
011...111
011...110
000...010
000...001
000...000
111...111
111...110
111...101
FS = ^VREF
2
ZS = 0
- FS = ^-VREF
2
1 LSB = ^VREF
4096
FULL-SCALE
TRANSITION
100...001
100...000
-FS
0
FS
DIFFERENTIAL INPUT
FS - 3/2 LSB
VOLTAGE (LSB)
Figure 9. Bipolar Transfer Function (MAX1279 Only)
How to Start a Conversion
An analog-to-digital conversion is initiated by CNVST,
clocked by SCLK, and the resulting data is clocked out on
DOUT by SCLK. With SCLK idling high or low, a falling
edge on CNVST begins a conversion. This causes the
analog input stage to transition from track to hold mode,
and for DOUT to transition from high impedance to being
actively driven low. A total of 16 SCLK cycles are required
to complete a normal conversion. If CNVST is low during
the 16th falling SCLK edge, DOUT returns to high imped-
ance on the next rising edge of CNVST or SCLK, enabling
the serial interface to be shared by multiple devices. If
CNVST returns high after the 14th, but before the 16th
SCLK rising edge, DOUT remains active so continuous
conversions can be sustained. The highest throughput is
achieved when performing continuous conversions. Figure
10 illustrates a conversion using a typical serial interface.
Connection to
Standard Interfaces
The MAX1277/MAX1279 serial interface is fully compati-
ble with SPI/QSPI and MICROWIRE (see Figure 11). If a
serial interface is available, set the CPU’s serial interface
in master mode so the CPU generates the serial clock.
Choose a clock frequency up to 24MHz.
SPI and MICROWIRE
When using SPI or MICROWIRE, the MAX1277/ MAX1279
are compatible with all four modes programmed with the
CPHA and CPOL bits in the SPI or MICROWIRE control
register. Conversion begins with a CNVST falling edge.
DOUT goes low, indicating a conversion is in progress.
Two consecutive 1-byte reads are required to get the full
12 bits from the ADC. DOUT transitions on SCLK rising
edges. DOUT is guaranteed to be valid t_DOUT later and
remains valid until t_DHOLD after the following SCLK rising
edge. When using CPOL = 0 and CPHA = 0, or CPOL = 1
and CPHA = 1, the data is clocked into the µP on the fol-
lowing rising edge. When using CPOL = 0 and CPHA = 1,
or CPOL = 1 and CPHA = 0, the data is clocked into the
µP on the next falling edge. See Figure 11 for connections
and Figures 12 and 13 for timing. See the Timing
Characteristics section to determine the best mode to use.
QSPI
Unlike SPI, which requires two 1-byte reads to acquire
the 12 bits of data from the ADC, QSPI allows the mini-
mum number of clock cycles necessary to clock in the
data. The MAX1277/MAX1279 require 16 clock cycles
from the µP to clock out the 12 bits of data. Figure 14
shows a transfer using CPOL = 1 and CPHA = 1. The
conversion result contains three zeros, followed by the
12 data bits, and a trailing zero with the data in MSB-
first format.
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