MAX1248 Datasheet, PDF (8/24 Pages) Maxim Integrated Products – 2x4-Channel, Simultaneous-Sampling 14-Bit DAS

+2.7V to +5.25V, Low-Power, 4-Channel,
Serial 10-Bit ADCs in QSOP-16
_______________Detailed Description
The MAX1248/MAX1249 analog-to-digital converters
(ADCs) use a successive-approximation conversion
technique and input track/hold (T/H) circuitry to convert
an analog signal to a 10-bit digital output. A flexible
serial interface provides easy interface to microproces-
sors (µPs). Figure 3 is a block diagram of the
MAX1248/MAX1249.
Pseudo-Differential Input
The sampling architecture of the ADC’s analog com-
parator is illustrated in the equivalent input circuit
(Figure 4). In single-ended mode, IN+ is internally
switched to CH0–CH3, and IN- is switched to COM. In
differential mode, IN+ and IN- are selected from two
pairs: CH0/CH1 and CH2/CH3. Configure the channels
with Tables 2 and 3. Please note that the codes for
CH0–CH3 in the MAX1248/MAX1249 correspond to
the codes for CH2–CH5 in the eight-channel
(MAX148/MAX149) versions.
In differential mode, IN- and IN+ are internally switched
to either of the analog inputs. This configuration is
pseudo-differential to the effect that only the signal at
IN+ is sampled. The return side (IN-) must remain sta-
ble within ±0.5LSB (±0.1LSB for best results) with
respect to AGND during a conversion. To accomplish
this, connect a 0.1µF capacitor from IN- (the selected
analog input) to AGND.
During the acquisition interval, the channel selected as
the positive input (IN+) charges capacitor C_HOLD. The
acquisition interval spans three SCLK cycles and ends
on the falling SCLK edge after the last bit of the input
control word has been entered. At the end of the acqui-
sition interval, the T/H switch opens, retaining charge
on C_HOLD as a sample of the signal at IN+.
The conversion interval begins with the input multiplex-
er switching C_HOLD from the positive input (IN+) to the
negative input (IN-). In single-ended mode, IN- is sim-
ply COM. This unbalances node ZERO at the compara-
tor’s input. The capacitive DAC adjusts during the
remainder of the conversion cycle to restore node
ZERO to 0V within the limits of 10-bit resolution. This
action is equivalent to transferring a charge of 16pF x
[(V_IN+) - (V_IN-)] from C_HOLD to the binary-weighted
capacitive DAC, which in turn forms a digital represen-
tation of the analog input signal.
Track/Hold
The T/H enters its tracking mode on the falling clock
edge after the fifth bit of the 8-bit control word has been
shifted in. It enters its hold mode on the falling clock
edge after the eighth bit of the control word has been
shifted in. If the converter is set up for single-ended
inputs, IN- is connected to COM, and the converter
samples the “+” input. If the converter is set up for dif-
ferential inputs, IN- connects to the “-” input, and the
difference of |IN+ - IN-| is sampled. At the end of the
conversion, the positive input connects back to IN+,
and C_HOLD charges to the input signal.
The time required for the T/H to acquire an input signal
is a function of how quickly its input capacitance is
charged. If the input signal’s source impedance is high,
CS ¹⁵
SCLK ¹⁶
DIN ¹⁴
SHDN ⁷
CH0 ²
CH1 ³
CH2 ⁴
CH3 ⁵
COM ⁶
REFADJ ⁹
VREF ⁸
INPUT
SHIFT
REGISTER
CONTROL
LOGIC
INT
CLOCK
ANALOG
INPUT
MUX
OUTPUT
SHIFT
REGISTER
T/H
CLOCK
IN _SAR
ADC
OUT
REF
+1.21V
REFERENCE
(MAX1248)
A ≈ 2.06*
20k
+2.500V
MAX1248
MAX1249
12 _DOUT
13 _SSTRB
1 _VDD
11 _DGND
10 _AGND
*A ≈ 2.00 (MAX1249)
Figure 3. Block Diagram
CAPACITIVE DAC
VREF
CH0
INPUT
MUX
C_HOLD
–+
COMPARATOR
ZERO
16pF
CH1
R_IN
C_SWITCH
9k
CH2
HOLD
TRACK
AT THE SAMPLING INSTANT,
CH3
T/H
THE MUX INPUT SWITCHES
SWITCH
FROM THE SELECTED IN+
CHANNEL TO THE SELECTED
COM
IN- CHANNEL.
SINGLE-ENDED MODE: IN+ = CHO–CH3, IN- = COM.
DIFFERENTIAL MODE: IN+ AND IN- SELECTED FROM PAIRS OF
CH0/CH1 AND CH2/CH3.
Figure 4. Equivalent Input Circuit
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