ZMD31020
Differential Sensor Signal Conditioner
1.
General Description
ZMD31020 is a signal conditioner for sensors (sensor elements; transducers) with differential output signal, e.g.
for Wheatstone-bridge-type sensors. The device provides digital correction and compensation of sensor offset,
gain, temperature sensitivity and non-linearity through an on-chip RISC-Microcontroller running a correction
algorithm.
A bidirectional digital serial interface allows for a simple PC-controlled calibration procedure, encompassing
reading of non-corrected sensor signal and temperature values and writing and programming of a resulting
calculated set of calibration coefficients into an on-chip parameter EEPROM. Thus a specific sensor and a
ZMD31020 conditioner device are mated digitally: fast, precise and without cost overhead for trimming
components and equipment.
ZMD31020 has been designed in 0.8µm EEPROM-CMOS for a typical supply voltage of 5V and an operating
temperature range –40°C ... +125°C, covering commercial, industrial and automobile applications. The device is
available both unpackaged as tested die or as finished product in 5.3mm width SSOP14.
A demokit including samples, documentation and PC-compatible hardware and software for emulation and
calibration is available.
2.
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Features
Optimized for ratiometric differential sensors
Cost-effective: a single 12-bit input ADC, 16-bit RISC-µC, 11-bit output DAC; no adjustment DACs needed
Minimum number of external components required: supply capacitor; sensor; analog output load capacitor
Temperature sensing optionally through off-chip or on-chip diode
Analog input multiplexer for differential sensor signal and temperature
Chopper-stabilized PGA, programmable to 3 differential gains (15.66, 24 and 42)
ADC resolves sensor signal with 12 bits, temperature with 10 bits
ADC’s output programmable to 4 zero-input bias values: 1/16, 1/8, ¼, ½ of conversion range
Analog input stage measures sensor signal ratiometrically, however temperature BG-related
Correction Processor: 16-bit ALU & (16 x 16 bit ) RAM; (1k x 16)-bit instruction ROM; (12 x 16)-bit
parameter EEPROM
Cancellation of chip-related offset in sensor and temperature signal through short-circuit input switch and
subtraction routine
Correction formula based on 7 calibration coefficients
Parameter EEPROM stores: configuration word, calibration coefficients, upper and lower output signal
limits, customer specific identifiers
Corrected sensor signal available both as 12-bit digital word at the I2C interface and as ratiometric analog
voltage from an 11-bit output DAC
Cycle time: 10ms. Response time: 11ms
Calibration of a sensor element / ZMD31020 combination to a desired output characteristic
through measurement of 7 uncorrected sensor and temperature value pairs
These values are read over the I2C interface and processed to calculate the 7 calibration coefficients.
Mating is completed by programming the calibration coefficients into the EEPROM over the I2C interface
PC-compatible hardware and software supporting the calibration procedure is available and included in the
demokit ZMD31020DK
Accuracy: ± 0.25% FSO typically
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Datasheet, Rev. 1.4, March 27th, 2002
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ZMD31020
Differential Sensor Signal Conditioner
3.
Application Circuit
220nF
+5V typ.
VDDA
VTN
Temperature
Sensing
Diode
VDDB1 (*)
Differential
Sensor
VDDB2 (*)
VDD
VPP
SCL
SDA
VBP
VOUT
10 to 25nF
VBN
VSSB
VSSA
VSS
0V
(*) either pin/pad may be chosen, whichever is more favourable layoutwise
Datasheet, Rev. 1.4, March 27th, 2002
2/21
ZMD31020
Differential Sensor Signal Conditioner
4.
Pin Description
PIN
Number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
(*)
(**)
Name
VOUT
VDDA (*)
VDD
VSS
SCL
SDA
VPP
VBN
VDDB2 (*)
VTN
VDDB1 (*)
VBP
VSSB (**)
VSSA (**)
Description
analog conditioned sensor signal output
analog device functions positive supply
digital device functions positive supply
digital device functions negative supply
I²C clock input, on-chip pull-up resistor
I²C data input / output, on-chip pull-up resistor
positive EEPROM programming voltage
differential sensor signal negative input
positive supply for sensor and temperature sensing diode
input for temperature sensing diode
positive supply for sensor and temperature sensing diode
differential sensor signal positive input
sensor negative supply
analog device functions negative supply
VDDA, VDDB1 and VDDB2 tied to common on-chip positive supply rail
VSSA and VSSB tied to common on-chip negative supply rail
Datasheet, Rev. 1.4, March 27th, 2002
3/21
ZMD31020
Differential Sensor Signal Conditioner
5.
Block Schematic
Datasheet, Rev. 1.4, March 27th, 2002
4/21
ZMD31020
Differential Sensor Signal Conditioner
6.
6.1
Functional Description
Configuration Word
Many of the following sections, describing each block of ZMD31020 in detail, will refer to configuration bits, part
of the configuration word stored under address (9)hex of the parameter EEPROM, see section 6.6.
These bits are settings for a number of on-chip device functions and select specific functional or parametrical
behaviour.
As described earlier the contents of the parameter EEPROM are determined and calculated, written and stored
under PC-control during the calibration procedure. Hence the configuration bits are coded and non-volatilely
stored once calibration of a ZMD31020 device / sensor pair has taken place, and will remain unchanged during
regular sensing operation, unless re-calibration is performed.
15
-
14
-
13
-
12
-
11
-
10
-
9
-
8
-
7
-
6
CH
5
TS
4
BP
3
G1
2
G0
1
O1
0
O0
Configuration word, stored under address (9)hex of the parameter EEPROM
Only 7 bits of the configuration word are relevant settings as follows:
Bit 0, Bit 1
à
O0, O1: select ADC’s output bias @ input zero
Bit 2, Bit 3
à
G0, G1: select PGA’s gain
Bit 4
à
BP: cross-switches differential sensor inputs VBP and VBN
Bit 5
à
TS: selects on-chip vs. off-chip temperature sensor
Bit 6
à
CH: enables PGA’s chopper-stabilization
A truth table, listing the code options of the individual configuration bit(s), is included in the section describing
the specific function which it (they) is (are) relevant for.
6.2
Differential Sensor
ZMD31020 has been specifically designed to be a signal conditioner for ratiometric differential sensors, e.g.
Wheatstone bridge type sensors.
The sensor is supplied from VDDB1 or VDDB2 (whichever pin/pad is more favourable layoutwise) at the + side
and tied to VSSB at the – side. The sensor's differential output signal is routed to VBP and VBN.
A ratiometric sensor typically generates a differential output signal proportional to the supply voltage applied to
it.
Sensor and signal conditioner ZMD31020 have the same supply (see block schematic in section 5), hence the
differential input voltage seen by ZMD31020 is ratiometric to it’s supply voltage.
6.3
Temperature Sensing
The transducer characteristic of a sensor tends to change with temperature.
To compensate for this, ZMD31020 is equipped to measure temperature, be it through an off-chip diode,
typically in close thermal contact with the sensor, or alternatively through an on-chip diode.
Datasheet, Rev. 1.4, March 27th, 2002
5/21
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