+2.7V, Low-Power, 4-Channel,
Serial 12-Bit ADCs in QSOP-16
MAX1246/MAX1247
Table 1. Control-Byte Format
BIT 7
(MSB)
START
BIT
7(MSB)
6
5
4
3
BIT 6
SEL2
NAME
START
SEL2
SEL1
SEL0
UNI/BIP
BIT 5
SEL1
DESCRIPTION
The first logic “1” bit after
CS
goes low defines the beginning of the control byte.
These three bits select which of the four channels are used for the conversion (Tables 2 and 3).
BIT 4
SEL0
BIT 3
UNI/BIP
BIT 2
SGL/DIF
BIT 1
PD1
BIT 0
(LSB)
PD0
1 = unipolar, 0 = bipolar. Selects unipolar or bipolar conversion mode. In unipolar mode, an
analog input signal from 0V to VREF can be converted; in bipolar mode, the signal can range
from -VREF / 2 to +VREF / 2.
1 = single ended, 0 = differential. Selects single-ended or differential conversions. In single-
ended mode, input signal voltages are referred to COM. In differential mode, the voltage
difference between two channels is measured (Tables 2 and 3).
Selects clock and power-down modes.
PD1
PD0
Mode
0
0
Full power-down
0
1
Fast power-down
1
0
Internal clock mode
1
1
External clock mode
2
SGL/DIF
1
0(LSB)
PD1
PD0
allowed between conversions. The acquisition time,
t
ACQ
, is the maximum time the device takes to acquire
the signal, and is also the minimum time needed for the
signal to be acquired. It is calculated by the following
equation:
t
ACQ
= 9 x (R
S
+ R
IN
) x 16pF
where R
IN
= 9kΩ, R
S
= the source impedance of the
input signal, and t
ACQ
is never less than 1.5µs. Note
that source impedances below 1kΩ do not significantly
affect the ADC’s AC performance.
Higher source impedances can be used if a 0.01µF
capacitor is connected to the individual analog inputs.
Note that the input capacitor forms an RC filter with the
input source impedance, limiting the ADC’s signal
bandwidth.
Analog Input Protection
Internal protection diodes, which clamp the analog input
to V
DD
and AGND, allow the channel input pins to swing
from AGND - 0.3V to V
DD
+ 0.3V without damage.
However, for accurate conversions near full scale, the
inputs must not exceed V
DD
by more than 50mV or be
lower than AGND by 50mV.
If the analog input exceeds 50mV beyond the sup-
plies, do not forward bias the protection diodes of
off channels over 4mA.
How to Start a Conversion
Start a conversion by clocking a control byte into DIN.
With
CS
low, each rising edge on SCLK clocks a bit from
DIN into the MAX1246/MAX1247’s internal shift register.
After
CS
falls, the first arriving logic “1” bit defines the
control byte’s MSB. Until this first “start” bit arrives, any
number of logic “0” bits can be clocked into DIN with no
effect. Table 1 shows the control-byte format.
The MAX1246/MAX1247 are compatible with SPI™/
QSPI™ and Microwire™ devices. For SPI, select the
correct clock polarity and sampling edge in the SPI
control registers: set CPOL = 0 and CPHA = 0. Micro-
wire, SPI, and QSPI all transmit a byte and receive a
byte at the same time. Using the
Typical Operating
Input Bandwidth
The ADC’s input tracking circuitry has a 2.25MHz
small-signal bandwidth, so it is possible to digitize
high-speed transient events and measure periodic sig-
nals with bandwidths exceeding the ADC’s sampling
rate by using undersampling techniques. To avoid
high-frequency signals being aliased into the frequency
band of interest, anti-alias filtering is recommended.
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