Datasheet

MAX1377/MAX1379/MAX1383
Dual, 12-Bit, 1.25Msps Simultaneous-Sampling
ADCs with Serial Interface
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Detailed Description
The MAX1377/MAX1379/MAX1383 use an input track
and hold (T/H) and SAR circuitry to convert an analog
input signal to a digital 12-bit output. The dual serial
interface requires a minimum of three digital lines
(SCLK, CNVST, and DOUT) and provides easy interfac-
ing to microprocessors (µPs) and DSPs. Four digital
lines are required for dual-output mode.
Input T/H Circuit
Upon power-up, the input T/H circuit enters its tracking
mode immediately. Following a conversion, the T/H
enters the tracking mode on the 14th SCLK rising edge
of the previous conversion (Figure 6). The T/H enters the
hold mode on the falling edge of CNVST. The time
required for the T/H to acquire an input signal is deter-
mined by how quickly the input capacitance is charged.
If the input signal’s source impedance is high, the acqui-
sition time lengthens. For the MAX1377/MAX1379, the
acquisition time, t
ACQ
, is the minimum time needed for
the signal to be acquired (see the Definitions section).
t
ACQ
is calculated by the following equation:
t
ACQ
≥ 9 x (R
S
+ R
IN
) x C
IN
(MAX1377/MAX1379)
where R
IN
= 450Ω, C
IN
= 16pF, and R
S
is the source
impedance of the input signal.
Figure 1 shows the acquisition time as tested using the
circuit of Figure 2. The acquisition time is the time
between the rising edge of a 1V to 3V step input and
the falling edge of CONVST which produced a stable
sample. Rs represents the source impedance of the
function generator (50Ω) and Rx represents the vari-
able filter resistance.
For the MAX1383, t
ACQ
has a typical constant value of
125ns. Also, it has a typical constant input impedance
of 11kΩ. Since the input voltage seen at the pin is a
function of a resistive voltage divider i.e., V
IN
x R
IN
/(R
IN
+ R
X
) = V
IN
x 11kΩ/(11kΩ + R
X
), it is very important to
select an R
X
<< 11kΩ to avoid large gain error.
MAX1377/MAX1379 Unipolar Mode
The MAX1377/MAX1379 support two simultaneously
sampled, single-ended conversions in unipolar mode.
Drive U/B low for unipolar mode. In unipolar mode,
switches A–D in Figure 3a close according to the posi-
tion of SEL. Drive SEL low to close switches A and D
and designate AIN1A and AIN2A as the active, single-
ended inputs referenced to RGND. Drive SEL high to
close switches B and D and select AIN1B and AIN2B
as the active, single-ended inputs referenced to RGND.
The output code in unipolar mode is straight binary.
See Figure 4a for the unipolar transfer function.
MAX1377/MAX1379 Bipolar Mode
Drive U/B high to configure the inputs for bipolar/differ-
ential mode. Switches A and C in Figure 3a are closed,
designating AIN1A (AIN2A) and AIN1B (AIN2B) as the
active, differential inputs. In bipolar mode, SEL is
ignored. The output code is in two’s complement.
Figure 5 shows the transfer function for bipolar mode.
MAX1383 Input Mode
A ±10V input mode is available on the MAX1383. It is
accomplished by utilizing a resistive divider on the
input followed by a low distortion amplifier to drive the
track and hold circuit. Special high voltage ESD struc-
tures are also utilized on these channels. When using
RGND
A
B
C
D
AIN1A
(AIN2A)
AIN1B
(AIN2B)
C
IN
R
IN
TO ADC+
TO ADC-
C
IN
R
IN
Figure 3a. MAX1377/MAX1379 Equivalent Input Circuit
ADC
RxRs
CONVST
C
1V TO 3V
STEP
Figure 2. Test Circuit
MAX1377 fig01
SOURCE IMPEDANCE, Rx (Ω)
ACQUISITION TIME (ns)
15010050
200
400
600
800
1000
1200
1400
1600
1800
0
0200
C = 1nF
C = 120pF
Figure 1. MAX1377/MAX1379 Acquisition Time vs. Source
Impedance