Model RAD242 is an intelligent, two-differential-channel, 24-bit analog-input interface unit that includes a Delta-Sigma A/D converter. The converter has a digital output filter with adjustable notch frequency from 10 Hz to 2000 Hz. The pod also has 13 bits of digital input/output. A precision 10 VDC voltage that can be used for transducer excitation is available at connector pin 5. As much as 60 mA current can be provided. It is packaged in a NEMA 4 enclosure for remote installation in harsh environments. RAD242 also provides a user-definable interrupt input and counter/timer input. An optional RAD Firmware Development Kit is available if you wish to use those user-programmable input lines or to develop a custom program.
A built-in watchdog timer resets the pod if, for some unexpected reason, the microcontroller “hangs up” or if the power supply voltage drops below 4.75 VDC. Data collected by the pod is stored in local RAM for later access through the computer’s serial port. This feature facilitates a stand-alone mode of operation. For example, a portable or laptop computer that has an RS-485 port can be brought to the pod, connected, and collect the data.
RAD242 is useful for wide dynamic range measurement of low-voltage analog signals. Typical applications include strain-gage weight measurements and precision thermometry. With the addition of the rugged enclosure and built-in intelligence, the RAD242 is ideal for process control, plant monitoring, and HVAC applications. RAD242’s design is optimized to perform readings on channel 0 with occasional readings on channel 1. This is admirably suited for thermocouple temperature measurements with the cold-junction reference attached to channel 1. In the case of sensors connected in a bridge, channel 1 can be used to monitor the excitation voltage or current in order to perform a ratiometric measurement.
There is a switchable current source on analog channel 0 that may be used to detect a burned-out or disconnected external sensor. This current source may be used to check that a sensor is OK before taking measurements from that sensor.
TRANSDUCER EXCITATION VOLTAGE
RAD242 includes a source for a precision 10 VDC that can be used to excite sensors such as strain gage load cells, pressure transducers, etc. As much as 60 mA current can be provided.
Model RAD242 uses a Delta-Sigma analog-to-digital converter and provides exceptional noise, linearity, and resolution characteristics. Delta-Sigma converters are fundamentally different from other converter technologies. Traditional A/D designs are based on high-accuracy analog components like resistors and comparators to provide reliable conversions. But analog circuits are subject to drift due to temperature and aging effects. Delta-Sigma converters are almost entirely digital and avoid many of the problems of analog circuits.
A delta-sigma converter consists of a sample/hold amplifier, a differential amplifier (subtractor, the delta part), an analog low-pass filter (integrator, the sigma part), a comparator, a 1-bit DAC, and a digital filter. The analog signal is applied to the subtractor along with the output of the l-Bit DAC. The subtractor output is applied to the low-pass filter and the low-pass-filtered output is, in turn, coupled to the comparator. The comparator output is samples of the difference signal at a frequency many times the Nyquist frequency. (The Nyquist frequency is equal to 2X the highest frequency present in the analog signal.) This process is called oversampling.
The delta-sigma converter in RAD242 samples the analog signal at 19.5 KHz or greater (depending on the gain selected). As a result, the quantization noise is spread over a wider frequency than the band of interest. The noise in the band of interest is reduced further by the low frequency filter. The output of the comparator provides the digital input to the 1-Bit DAC so that the system functions as a negative feedback loop that tries to minimize the difference signal. The digital data that represents the analog input voltage is contained in the duty cycle of the pulse train appearing at the output of the comparator. If the analog signal is zero, the duty cycle is 50%. The duty cycle increases for a positive analog signal and decreases for a negative one. The digital filter removes noise injected during the conversion process. The output of the digital filter is a binary representation of the rolling average of the sampled signal.
This architecture is inherently monotonic (no missing codes) and provides very high accuracy but at the expense of measurement speed (data output rate). For example, to achieve 22 bits resolution, a 10 MHz clock rate would result in a data or measurement rate of less than 10 Hz. The processes of oversampling and then integrating the readings produces a fixed delay from the time that an analog sample is taken until the digital value is output. The amount of delay is dependent on the oversampling rate. In most signal processing applications this lag is not significant. However, for real-time control applications, this lag should be considered.
Despite the use of inherently stable delta-sigma converters with low-drift components an occasional calibration may be necessary. Calibration removes offset and gain errors due to the effects of temperature drift, supply changes, zero drift, and full scale gain errors. A calibration routine may be required whenever there is a significant change in the ambient operating temperature or supply voltage. It should also be initiated if there is a change in the selected channel gain, filter notch or bipolar/unipolar input range.
Calibration of RAD242 can be done in any of three ways:
a. System calibration.
b. Self calibration of the A/D.
c. Background calibration of the A/D.
System calibration requires application of zero volts and full-scale voltage to the pod’s analog inputs. Self calibration of the A/D chip can be performed in software and uses the reference voltage. Background calibration of the A/D can be commanded in software to occur with each conversion. In the background-calibration mode, the calibration procedure is interleaved with the normal conversion sequence and reduces the conversion rate by a factor of six. Calibration constants are stored automatically in the EEPROM.
PROGRAMMABLE DIGITAL FILTER
The internal digital filter removes noise added during the conversion and sums the oversampled readings to produce multiple bit measurements at a data update rate determined by the first notch of the digital filter. Internally, the converter provides measurements at a 19.5 KHz output rate and the data is filtered down to the update rate defined by the digital filter. Post filtering may be used to increase the update rate or to reduce output noise. For example, if the bandwidth is 7.86 Hz at the desired resolution (resulting in an update rate of 30 Hz) but the desired update rate is 100 Hz, the data could be taken at the 100 Hz rate and filtered down to the 7.86 Hz bandwidth to achieve the same accuracy.
Post filtering can also be used to reduce the output noise for bandwidths below 2.62 Hz. At a gain of 128, the output RMS noise is 250 nV. This is essentially device noise or white noise, and since the converter uses a chopper-stabilized input stage to reduce input drift, the noise has a flat frequency response. By reducing the bandwidth below 2.62 Hz, the noise in the resultant passband can be reduced. A reduction in bandwidth by a factor of two results in a reduction in the output rms noise equal to the square root of two. This additional filtering will also result in a longer settling time.
The worst-case settling time of the filter for a full-scale step input change is four times the data rate. For example, with the first filter notch at 50 Hz, the full-scale step settling time of the filter would be 80 mS (maximum). Due to a digital signal processing method used by the delta-sigma converter in the RAD242, changing the channel gain (in addition to the output sample rate) impacts the resolution.
In addition to the analog input channels, RAD242 has 13 bits of bidirectional digital input/output bits. In the output mode, there is high current-sink capacity (350 mA) and the open-collector outputs can be connected to external voltage sources of up to 50 V. If no external source is used then each I/O bit is pulled up to +5 VDC.