MAX4578/MAX4579
High-Voltage, Single 8-to-1/
Dual 4-to-1 Cal-Multiplexers
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Detailed Description
The MAX4578/MAX4579 are multiplexers with addition-
al calibration features. Internal resistor-dividers gener-
ate accurate voltage ratios from an external voltage
reference, allowing zero and full-scale calibration of
ADC systems, as well as facilitation of system self-mon-
itoring. To access the resistor-dividers, assert the CAL
pin. When CAL and ENABLE are asserted, the three
address pins select one of the various resistor-divider
or external reference outputs. The MAX4578/ MAX4579
also contain a LATCH input that allows the state of the
CAL and address signals to be captured.
Calibration Functions
The gain-divider, offset-divider, REFHI, and REFLO
modes allow calibration of offset and gain errors in
ADC systems. The gain-divider mode outputs a voltage
ratio that is 4081/4096 of V
REFHI
- V
REFLO
, accurate to
0.1/4096 or better than 15 bits. The offset-divider mode
outputs a voltage ratio that is 15/4096 of V
REFHI
-
V
REFLO
, also accurate to 0.1/4096. The REFHI mode
allows the voltage on the REFHI pin to be switched to
the output. The REFLO mode allows the voltage on the
REFLO pin to be switched to the output.
Self-Monitoring Functions
The self-monitoring functions are intended to allow an
ADC to measure its own supply voltage. The MAX4578
has an internal divide-by-two resistor string between V+
and GND that is accurate to 8 bits. It also has a 5/8
resistor string between V+ and V- that is accurate to 8
bits. This divider string allows measurement of the nega-
tive supply with a unipolar ADC. GND can also be
switched to the output, eliminating the need for an addi-
tional multiplexer channel.
__________ Applications Information
The MAX4578/MAX4579’s construction is typical of
most CMOS analog switches. There are three supply
pins: V+, V-, and GND. The positive and negative
power supplies provide drive to the internal CMOS
switches and set the limits of the analog voltage on any
switch. Reverse-biased ESD protection diodes are
internally connected between each analog signal pin
and both V+ and V-. If the voltage on any pin exceeds
V+ or V-, one of these diodes will conduct. During nor-
mal operation, these reverse-biased ESD diodes leak,
forming the only current drawn from V-.
Virtually all the analog leakage current is through the
ESD diodes. Although the ESD diodes on a given sig-
nal pin are identical, and therefore fairly well balanced,
they are reverse-biased differently. Each is biased by
either V+ or V- and the analog signal. This means their
leakage varies as the signal varies. The difference in
the two-diode leakage from the signal path to the V+
and V- pins constitutes the analog signal-path leakage
current. All analog-leakage current flows to the supply
terminals, not to the other switch terminal, which explains
how both sides of a given switch can show leakage cur-
rents of either the same or opposite polarity.
There is no connection between the analog-signal
paths and GND. The analog-signal paths consist of an
N-channel and P-channel MOSFET with their sources
and drains paralleled and their gates driven out of
phase with V+ and V- by the logic-level translators.
V+ and GND power the internal logic and logic-level
translators and set the input-logic thresholds. The logic-
level translators convert the logic levels to switched V+
and V- signals to drive the gates of the analog switch-
es. This drive signal is the only connection between the
logic supplies and the analog supplies. All pins have
ESD protection to V+ and to V-.
Increasing V- has no effect on the logic-level thresh-
olds, but it does increase the drive to the P-channel
switches, which reduces their on-resistance. V- also
sets the negative limit of the analog-signal voltage.
The logic-level thresholds are CMOS- and TTL-
compatible when V+ is greater than +4.5V.
Bipolar-Supply Operation
The MAX4578/MAX4579 operate with bipolar supplies
between ±4.5V and ±20V. The V+ and V- supplies need
not be symmetrical, but their sum cannot exceed the
absolute maximum rating of 44V. Note: Do not connect
the MAX4578/MAX4579 V+ pin to +3V AND connect
logic-level input pins to TTL logic-level signals. TTL
logic-level outputs can exceed the absolute maximum
ratings, which will cause damage to the part and/or
external circuits.
Single-Supply Operation
The MAX4578/MAX4579 operate from a single supply
between +4.5V and +36V when V- is connected to GND.
All of the bipolar precautions must be observed (see
Bipolar Supply Operation
section). However these parts
are optimized for ±15V operation, and most AC and DC
characteristics are degraded significantly when depart-
ing from ±15V. As the overall supply voltage (V+ to V-) is
lowered, switching speed, on-resistance, off-isolation,
and distortion will degrade, and supply current will
decrease (see the
Typical Operating Characteristics
section).
