Abstract: IC switches and multiplexers are proliferating, thanks to near-continual progress in lowering the supply voltage, incorporating fault-protected inputs, clamping the output voltage, and reducing the switch resistances. The latest of these advances is the inclusion of precision resistors to allow two-point calibration of gain and offset in precision data-acquisition systems.
Called calibration multiplexers
(cal-muxes), these new devices are exemplified by a low-voltage, 8-channel
CMOS multiplexer (MAX4539). It has internal precision resistive dividers that
generate accurate voltage ratios, either from an external reference or from
its own supply voltage. By eliminating the need for external resistor strings,
multiplexers, and logic gates, the MAX4539 provides an accurate and convenient
means for calibrating and monitoring A/D converters.
The MAX4539 operates from a single
supply of 2.7V to 12V or a dual supply in the range ±2.7V to ±6V.
Switch on-resistances measure 100ohm, matched to within 6ohm maximum. They handle rail-to-rail
analog signals and exhibit off leakages of only 0.1nA over the industrial
temperature range. Package options include a 20-pin SO, a 20-pin DIP, and
the small 20-pin SSOP.
The internal resistors are accessed
and configured via a versatile digital interface that includes a 3-bit address,
enable input, calibration input, and latch input. In turn, these inputs drive
an internal, 16-output logic decoder that controls both the main multiplexer
and the switches that configure a calibration.
The MAX4539 turns on when ENABLE
is high and behaves as a conventional multiplexer unless the CAL input is
asserted. With CAL and ENABLE both asserted, the three address inputs (via
the logic decoder) select one of the resistor-divider or external-reference
outputs. The LATCH function lets the chip capture this state, thereby releasing
the address bus.
Two main features set the cal-mux
apart. One is the LATCH function, and the other is its capability for calibration
and self-monitoring. Four internal resistor dividers give access to four fixed
ratios: (15/4096)(VREFHI -VREFLO) and (4081/4096)(VREFHI
-VREFLO) (where VREF is the external reference), (5/8)(V+
-V-), and (1/2)(V+ -VGND). In addition to
these quantities, the MAX4539 gives individual access to GND, REFHI, and REFLO.
The use of precision internal resistors with excellent thermal tracking results
in a calibration procedure with accuracies better than 15 bits (0.1/4096)
over the industrial temperature range.
Each address gives a different
configuration of the multiplexer and calibration switches. Driving the LATCH
input high captures a given control state, enabling the device to ignore perturbations
on the address lines until LATCH returns low.
The MAX4539 is very useful in multi-input
industrial control systems that employ A/D converters (Figure 1). By
generating reference voltages that are converted by the ADC and recorded by
the microcontroller, it can null the two major errors associated with ADC
systems (offset error and gain error). Figure 2 illustrates the operating
sequence for such a procedure in a single-supply system.
Figure 1. The MAX4539 cal-mux simplifies calibration of a multi-channel industrial control system.
Figure 2. This flow chart details a calibration procedure implemented in the Figure 1 system.
First, the cal-mux applies one-half
the supply voltage as a first verification that proper power is applied. The
system then measures zero offset and gain error, and forms an equation to
correct the subsequent readings. To calibrate for offset error--the input
voltage (ideally zero) necessary to produce an all-zero digital output--the
cal-mux applies (15/4096)(VREFHI -VREFLO). Using a 12-bit
ADC with 4.096V reference as an example, (15/4096)(VREFHI -VREFLO)
equals 15mV and also 15LSBs. The digital output should therefore be 000000001111.
To measure offset error, the microcontroller simply records the difference
between the ADC's digital output and 000000001111.
Gain error is measured by applying
(4081/4096)(VREFHI -VREFLO). The microcontroller then
records the difference between the ADC's digital output and 111111110000.
Knowing the ADC's offset error and gain error, system software constructs
calibration factors that adjust the subsequent outputs to produce correct
readings.
Finally, the cal-mux monitors the
system power supply by generating proportional voltages that are easily converted
by the ADC and measured by the microcontroller: (1/2)(V+ -VGND)
for single-supply systems, and (5/8)(V+ -V-) for dual-supply
systems.
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