Rainbow Electronics MAX1139 User Manual
Page 10
MAX1136–MAX1139
signal. At the end of the acquisition interval, the T/H
switches move to the hold position retaining the charge
on C
T/H
as a stable sample of the input signal.
During the conversion interval, the switched capacitive
DAC adjusts to restore the comparator input voltage to
0V within the limits of 10-bit resolution. This action
requires 10 conversion clock cycles and is equivalent
to transferring a charge of 11pF
✕
(V
IN+
- V
IN-
) from
C
T/H
to the binary weighted capacitive DAC, forming a
digital representation of the analog input signal.
Sufficiently low source impedance is required to ensure
an accurate sample. A source impedance of up to 1.5k
Ω
does not significantly degrade sampling accuracy. To
minimize sampling errors with higher source impedances,
connect a 100pF capacitor from the analog input to GND.
This input capacitor forms an RC filter with the source
impedance limiting the analog-input bandwidth. For larg-
er source impedances, use a buffer amplifier to maintain
analog-input signal integrity and bandwidth.
When operating in internal clock mode, the T/H circuitry
enters its tracking mode on the eighth rising clock edge
of the address byte (see the Slave Address section). The
T/H circuitry enters hold mode on the falling clock edge of
the acknowledge bit of the address byte (the ninth clock
pulse). A conversion, or series of conversions, are then
internally clocked and the MAX1136–MAX1139 holds
SCL low. With external clock mode, the T/H circuitry
enters track mode after a valid address on the rising
edge of the clock during the read (R/W = 1) bit. Hold
mode is then entered on the rising edge of the second
clock pulse during the shifting out of the first byte of the
result. The conversion is performed during the next 10
clock cycles.
The time required for the T/H circuitry to acquire an
input signal is a function of the input sample capaci-
tance. If the analog-input source impedance is high,
the acquisition time constant lengthens and more time
must be allowed between conversions. The acquisition
time (t
ACQ
) is the minimum time needed for the signal
to be acquired. It is calculated by:
t
ACQ
≥ 9
✕
(R
SOURCE
+ R
IN
)
✕
C
IN
where R
SOURCE
is the analog-input source impedance,
R
IN
= 2.5k
Ω, and C
IN
= 22pF. t
ACQ
is 1.5/f
SCL
for internal
clock mode and t
ACQ
= 2/f
SCL
for external clock mode.
Analog Input Bandwidth
The MAX1136–MAX1139 feature input-tracking circuitry
with a 5MHz small-signal bandwidth. The 5MHz input
bandwidth makes it possible to digitize high-speed
transient events and measure periodic signals with
bandwidths exceeding the ADC’s sampling rate by
using under sampling techniques. To avoid high-fre-
quency signals being aliased into the frequency band
of interest, anti-alias filtering is recommended.
Analog Input Range and Protection
Internal protection diodes clamp the analog input to
V
DD
and GND. These diodes allow the analog inputs to
2.7V to 3.6V and 4.5V to 5.5V, Low-Power,
4-/12-Channel, 2-Wire Serial 10-Bit ADCs
10
______________________________________________________________________________________
TRACK
TRACK
HOLD
C
T/H
C
T/H
TRACK
TRACK
HOLD
AIN0
AIN1
AIN2
AIN3/REF
GND
ANALOG INPUT MUX
CAPACITIVE
DAC
REF
CAPACITIVE
DAC
REF
MAX1136
MAX1137
HOLD
HOLD
TRACK
HOLD
V
DD
/2
Figure 4. Equivalent Input Circuit