Calibration of isolated analog-to-digital converters

Details Calibration of isolated analog-to-digital converters Calibration of isolated analog-to-digital converters

2001-07-12

2002-09-03

JeanPierre, Peguy (Department: 2819)

Coded data generation or conversion

Converter compensation

C341S120000

Reexamination Certificate

active

06445315

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to analog-to-digital converters. In particular, the present invention is directed toward calibration of isolated analog-to-digital converters.
BACKGROUND OF THE INVENTION
Measurement data collected by isolated analog-to-digital converters (ADCs) in multiple data channels may be related. Data from the isolated ADCs may be transmitted to a microcontroller or programmable logic device for centralized processing. The gain and offset of individual channels relative to one another is an issue which may require attention. Such applications are precise, cost-sensitive applications where providing each ADC with a precise reference is not an affordable solution.
ADCs are known to need isolation in applications subject to different voltage levels. The reference voltage of an ADC is not necessarily the same as the reference voltage which would be observed by an outside observer (e.g., relative to absolute ground). Moreover, the recording equipment processor used with an ADC may work with another power supply, and the various power supplies may be very different both in voltage levels and in power supply characteristics.
For example, in a power meter chip, used to measure power consumption (e.g., in a residential or business metering or measuring system) with one or three phases, there may be provided a ground line and a live line. The live line may have a potential, for example, 220 Volts from the neutral line. Thus, in order to make a measurement on the ground or neutral line and simultaneously make a measurement on the live line, the ADC front end must be able to accommodate vastly different voltage levels (i.e., the voltage reference to the live line, such as 220 Volts absolute, and a ground reference). The measurement device and the data processor may need to be isolated electrically from one another.
It is known in the art to use isolation capacitors as an isolation barrier in multi-link systems. A transformer or other isolation device may also be used to isolate the ADC from the microcontroller or programmable logic device. However, such isolation barriers may make it difficult to calibrate the ADC since the microcontroller or programmable logic device may have difficulty sending a precise voltage reference signal through the isolation barrier.
FIG. 1
is a block diagram of a measurement system
9
of the prior art. Such measurement systems may be used to measure various analog parameters in environments where voltage isolation is required. For example, in residential power metering, a measurement device (front end) may be measuring power at line voltage (e.g., 220 Volts) and some form of isolation may be required to protect the user and processor (back end) which may be at a much lower potential. Similarly, in medical systems, voltage isolation may be required as a fail-safe to prevent a patient from being electrocuted due to potential differences between various medical monitoring devices connected to a patient.
Referring to
FIG. 1
, measurement system
9
may include a digital signal processor (DSP)
11
, link chip
12
, capacitor C
1
13
, analog-to-digital converter (ADC) and link chip
15
, and a sensor
16
. Sensor
16
may comprise any one of a number of known analog sensors for measuring a particular parameter (e.g., temperature, pressure, voltage, amperage, power consumption, or the like).
Analog-to-digital converter (ADC) and link chip
15
may convert the analog output of sensor
16
to a digital value (typically a one-bit data stream) and outputs this data stream to a digital signal processor (DSP)
11
via link chip
12
and isolation capacitor
13
. In addition to digital data values transmitted from analog-to-digital converter (ADC) and link chip
15
to digital signal processor (DSP)
11
, other signals may need to be exchanged between the two chips.
For example, clock signals and control signals (including calibration signals or voltage levels) may be transmitted from digital signal processor (DSP)
11
to analog-to-digital converter (ADC) through link chip
15
. In addition, digital signal processor (DSP)
11
may need to provide power supply voltage to analog-to-digital converter through link chip
15
. In the prior art, additional signal lines may be required for such additional signals, increasing the complexity and cost of the device.
In many applications it may be necessary to isolate analog-to-digital converter (ADC) from link chip
15
and digital signal processor (DSP)
11
due to differences in voltage potential. An isolation capacitor
13
may be employed to isolate the voltage potential between analog-to-digital converter and link chip
15
and digital signal processor (DSP)
11
.
FIG. 2
is a block diagram of another embodiment of a measurement system
19
of the prior art. Measurement system
19
includes a digital application specific integrated circuit (ASIC) or programmable logic device (PLD)
21
such as a digital signal processor and link chip, a resistor
22
, capacitor
23
, transformer
24
, analog-to-digital converter (ADC)
25
and capacitor
26
.
ASIC or PLD
21
may include a transmitter
27
and receiver
29
coupled to each other through switch
28
. Data may be selectively transmitted and received over the connection between ASIC or PLD
21
and ADC
25
. In addition, ASIC or PLD
21
may provide power to ADC
25
through this same link.
ADC
25
may include a diode
30
and a rectifier
31
. Signals from secondary winding
33
of transformer
24
may be rectified by rectifier
31
and diode
30
to produce a voltage a capacitor
26
which in turn is the power supply for ADC
25
.
As in the embodiment of
FIG. 1
, transmitter
27
may transmit to primary winding
32
of transformer
24
a square wave which may be partially blocked or distorted by capacitor
23
from transformer
24
. ADC
25
may detect a pause during the tri-state operation and takes over the data link, sending data and status back to receiver
29
. During this take-over period, however, voltage at power supply
26
may droop significantly if many bits are transmitted, and full logic levels may not re-establish themselves.
In addition, an isolated ADC may require an accurate low noise reference voltage from, for example, a microcontroller. If the ADC is rendered in CMOS, a superquality voltage reference may be required for the ADC to accurately measure analog values. CMOS circuitry may be more susceptible to drift due to temperature variations and the like, as well as initial accuracy of measurement.
Further, in order to perform an absolute accurate conversion with an isolated ADC, it may be necessary to send an accurate low noise reference voltage across the isolation barrier. If the ADC is rendered in CMOS, a superquality voltage reference may be required for the ADC to accurately measure analog values. CMOS circuitry may be more susceptible to drift due to temperature variations and the like, as well as initial accuracy of measurement. A better reference, therefore, may be implemented on the isolated side.
Moreover, in some applications, it may be necessary to provide multiple isolated ADCs with precisely matched gains for acquiring related signals such that conversion data are known to be exactly at the same scale. These may be ratiometric measurements between several isolated points. Prior art techniques may use separate chips for each ADC side to provide a reference signal. However, such a solution creates extra cost and increases complexity and size of the overall circuitry.
SUMMARY OF THE INVENTION
Related measurement data collected by isolated analog-to-digital converters in multiple channels may be transmitted to a microprocessor or programmable logic device for centralized processing, to eliminate selected gain and offset effects common to the different analog-to-digital converters in the different channels, eliminating the consequences of drift in the different channels.
In particular, a pair of precision resistors is provided to calibrate the different channels. The ADCs may be factory calibrated and th

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