ML2264
1.0 FUNCTIONAL DESCRIPTION
The ML2264 uses a two stage flash technique for A/D
conversion. This technique first performs a 4 bit flash
conversion on V
IN
to determine the 4 MSB’s. These 4
MSB’s are then cycled through an internal DAC to
recreate the analog input. This reconstructed analog input
signal from the DAC is then subtracted from the input, and
the difference voltage is converted by a second 4 bit flash
conversion, providing the 4 LSB’s of the output data word.
1.1 MULTIPLEXER ADDRESSING
The ML2264 contains a 4-channel single ended analog
multiplexer. A particular input channel is selected by using
the address inputs A0 and A1. The relationship between
the address inputs, A0 and A1, and the analog input
selected is shown in Table 1.
Selected
Analog Channel
A IN 1
A IN 2
A IN 3
A IN 4
Address Input
A0
0
1
0
1
A1
0
0
1
1
The equivalent input circuit for the converter is shown in
Figure 8. When the conversion starts in the T/H mode
(WRØ in the WR-RD mode or
RDØ
in the RD mode) S1,
S4 and S6 close and S3 opens. This period is known as the
acquisition period where the MSB flash converter tracks
the input signal and the LSB flash converter samples it.
During this period, V
IN
is connected to the 16 MSB and
15 LSB comparators. Thus 38pF of input capacitance must
be charged up through the combined R
ON
resistance of
the internal analog switches plus any external source
resistance, R
S
. In addition, there is a stray capacitance of
approximately 11pF that needs to be charged through the
external source resistance R
S
. This period ends in the WR-
RD mode when
WR
or by an internal timer in the RD
mode. At this point S1 and S4 open and the analog input
at V
IN
is no longer being sampled; thus during this time
the analog voltage on V
IN
does not affect converter
performance.
As shown above, the critical period for charging up the
analog input occurs when the MSB and LSB comparators
are sampling the input, known as the acquisition period.
The source of the external signal on V
IN
must adequately
charge up the analog voltage during the acquisition
period. To do this, the input must settle within the
required analog accuracy tolerance at least 50ns before
the end of the acquisition period so that the MSB
comparators have adequate time to make the correct
decision. If more time is needed due to finite charging or
settling time of the external source, the
WR
low period
can be extended in WR-RD mode. In RD mode, since the
acquisition time is fixed by internal delays, the burden is
on the external source to charge up and settle the input
adequately.
When the ML2264 operates in the S/H mode (pin 23 =
V
CC
) both the MSB and the LSB flash converter perform a
true sample and hold operation during the acquisition or
sampling period. This period starts after the falling edge of
INT
and ends with the falling edge of
WR
in the WR-RD
mode or the falling edge of
RD
in the RD mode. The
duration of this period is user controlled and must satisfy a
minimum of t
P
.
During this period S1, S3, S4 and S6 close, therefore
46pF of input capacitance must be charged up in addition
to the 11pF of stray capacitance.
1.3 TRACK AND HOLD vs. SAMPLE AND HOLD
The MSB Flash Converter of the ML2264 in T/H mode has
a track and hold mechanism for sampling the input. The
input is attached to the MSB comparators directly in the
MSB compare cycle, or acquisition period. When the MSB
compare cycle ends, the state of the MSB comparators is
latched. The LSB Flash Converter always performs a S/H
operation. Thus, the analog input signal can be changing
during the MSB compare cycle, or acquisition period, and
Table 1. Multiplexer Address Decoding
The address inputs are latched into the ML2264 on the
falling edge of the
RD, WR,
or
CS
depending on the state
of pins SH/TH and mode as shown in Table 2.
Address Latching
Signal
RDØ
WRØ
CSØ
CSØ
Mode
GND
V
CC
GND
V
CC
Table 2.
Operation Mode
GND
GND
V
CC
V
CC
In the Sample & Hold mode of operation
CS
is used as the
address latch enable, allowing for continuous conversions
without addressing a given analog input for each
conversion.
The Track & Hold mode of operation requires an analog
input to be addressed and latched for each conversion
that the ML2264 performs.
1.2 ANALOG INPUTS
The analog input on the ML2264 behaves differently than
inputs on conventional converters. The analog input
current requirements change while the conversion is in
progress, and the amount of input current depends on
what cycle the converter is in.
9
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
