02-21-2022, 04:36 PM
Here, we taker a closer look at the Lawson Labs Model 201, and analyze the various design considerations that went into maximizing the usable resolution. As the core of a data acquisition system, the Model 201 performs A/D conversion, data handling, and digital input and output. Expansion boards can be added for analog output or other special functions. The questions addressed here are directed towards what you actually need to make high resolution and accuracy practical.
Analog inputs
24 bit A/D chips that are intended for digital audio are not specified for DC accuracy. You may be able to get reasonable reproducibility out of an audio chip, but guaranteeing absolute accuracy is out of the question without over-the-top calibration procedures. The AD7712 is specified for DC accuracy, so it is a good choice for instrumentation. It has built in self-calibration circuitry, which we employ. However, the input impedance is not constant, but rather depends on the data rate because of capacitive sampling of the input. That sampling can interact with the input voltage source and reduce the real resolution and accuracy. Anyway, the input impedance is not nearly high enough to take full advantage of the accuracy and resolution of the A/D converter when used with higher impedance voltage sources.
That means a high-impedance buffer must be placed in front of the A/D converter. That buffer will introduce small offset and gain errors, so the the built-in self-calibration circuitry can no longer do the whole calibration job. We'll come back to that. The high-impedance buffer section needs to have excellent common mode rejection. Let's take a minute to explain what that means. A single-ended A/D input measures voltage in comparison to a ground potential, theoretically zero volts. The problem with that is that no two points can be relied on to be at the exact same potential. A 24-bit A/D converter with a +/-5 voltage range, like the Model 201, resolves less than 1 microvolt. It doesn't take much resistance or current flow to create a 1 uV voltage drop across a wire. There are also inevitable AC voltages present, but we talk about AC noise in other places, so will stick to the DC effects here. In order for the last few bits of a 24 bit A/D to have real meaning, you need to make a differential measurement, instead of relying on a hypothetical zero-volt ground. As you might guess, a differential measurement responds to the difference between the plus input and the minus input. For an ideal differential input, you could connect both inputs to a +/-5 volt sine wave and the reading should stay zero, regardless.
In practice, the gain from the minus side will not perfectly match the gain from the plus side, so some small remnant of the sine wave of common mode input voltage would show in your readings. We need that remnant to be vanishingly small in order to be able to ignore common mode effects. In the Model 201, the common mode rejection is minimized with a hardware trim. That leaves the offset and gain errors that occur in the added high impedance differential buffer section. You can try to trim out these errors in the hardware, but the process is necessarily imperfect, especially when you measure over a large temperature range. Also, offset and gain trims interact, and trimpots can change resistance over time and as the result of vibration. Best is to include the offset and gain errors in the self-calibration process. Again, that means the built-in self-calibration cannot do the whole job, if you are going to maintain the maximum DC accuracy.
A multiplexer is placed in front of the new amplifier so that calibration signals can run through the input buffer to the A/D. In the case of the Model 201, there is an 8-channel differential multiplexer with six channels for general-purpose input and two channels dedicated for calibration. All six available inputs have series protection resistors. These do not introduce noticeable error because of the extra-high input impedance buffers that follow. Then, the entire system is calibrated by both the internal self-calibration, and by the external signals, as managed by the external microcontroller.
To sum it up, the front end of the circuitry includes a protected multiplexer, a high-impedance differential amplifier stage, and calibration signals. The output of all that is fed to the A/D chip, under microprocessor control. Actually, there are two more circuit blocks between the input section and the A/D chip.
Overvoltage protection
Over enough time, all sorts of mishaps at the analog inputs are bound to occur. The easy way to protect the A/D chip is with clamp diodes to the supply rails. Clamp diodes can leak enough current to add measurable error to a high impedance input. Also, clamp diodes turn on incrementally over a range of voltage and may not save the A/D chip when the transient hits. A better clamp, for more reliability and best accuracy, requires a comparator and an analog switch to guarantee proper protective clamping.
Programmable filter
Delta sigma converters like the AD7712 are extremely effective at rejecting most frequencies of input noise. Still, for any given data rate, there are certain frequencies that elude the digital filtration. The solution to that problem is an analog low pass pre-filter that will remove the unwanted frequencies. Active filters introduce DC errors, so we avoid them. A simple one-pole RC filter will do the low pass job, but because the Model 201 is digitally programmable over a wide range of data rates, different filter constants are appropriate for different circumstances. So, the Model 201 has three programmable analog filter time constants. That added functionality involves adding a high-quality polypropylene filter capacitor and a mechanism to switch in different series resistors.
Power input and power supplies
The power input is given special treatment. First, we protect against reversed connections, then against overvoltage. Third, we pre-regulate at 24 volts, for any case where the input voltage is higher than that. Then, a 5 volt standby supply is produced. The standby supply is always on to keep the microcontroller active and checking for serial commands. If there is serial activity, the microcontroller powers up the rest of the power supply chain. The preconditioned input voltage of 14 to 24 volts is regulated at 12 volts. From that, a charge pump produces a -12 volt supply. Those +/- 12 volt supplies are regulated, but not precisely regulated. The +/- 12 volts is re-regulated to +/-6.2 volts to power the sensitive analog circuitry. An analog 5 volt supply is also derived, using a precision 5 volt reference that runs from the +12 volts. There are two more reference supplies needed by the A/D converter in order to take full advantage of its dynamic range. Those are at +/- 2.5 volts. Finally, a precision 5 volt reference output is provided for off-board circuitry.
It adds up to 12 power supply voltages total. In addition, various connections to the supplies are decoupled from each other with small-value series resistors and capacitor filters. A typical decoupling network might be a 10 ohm series resistor with a 10 uF and a 0.1 uF shunt capacitor. Decoupling networks eliminate unwanted interactions that can propagate via the power supplies.
To allow battery powered applications, total current drain must be kept low. The Model 201 draws just about 1.5 mA in sleep mode, and about 17 mA during normal operation.
Optical isolation
There are a few more power supply components on the Model 201 board, but the power for them comes from the host computer serial port. The RS232 interface provides plus and minus power to a chip which drives, and is driven by, the optocouplers for the serial communication. The reasons for needing this additional level of isolation are explained in detail in other posts on this discussion site. For now, just remember that isolation breaks ground loops and protects against potentially damaging fault currents.
The remaining circuit blocks on the Model 201 board are for digital input and output, and for expansion. There is also another layer of optical isolation for the four expansion outputs, A through D.
Overview of the microcontroller function:
Back in the '80s affordable microcontrollers did not include peripherals like serial interfaces. The PIC processor selected for the Model 201 was fast and efficient for its day, but unadorned with extra features. We built a stripped-down real-time operating system that prioritized serial communications and getting the analog data out of the A/D chip and into a small buffer. Digital input and output, plus housekeeping were handled as lower priority tasks. For data logging applications, the microcontroller can keep time and send back a pre-defined data set as fast as 1000 times per second or as slowly as once a day. Alternatively, it can passively respond to commands from the host computer.
The Model 201 is still an active product 35 years after its introduction, so it has proven itself in the marketplace. If you need just a little more than 16-bits of real resolution, you can get away with lot by starting with a 24-bit delta sigma A/D converter chip. But, if you want 22 or 23 bits of real, usable, reliable resolution, you need to do everything exactly right.
Tom Lawson
February 2022
Analog inputs
24 bit A/D chips that are intended for digital audio are not specified for DC accuracy. You may be able to get reasonable reproducibility out of an audio chip, but guaranteeing absolute accuracy is out of the question without over-the-top calibration procedures. The AD7712 is specified for DC accuracy, so it is a good choice for instrumentation. It has built in self-calibration circuitry, which we employ. However, the input impedance is not constant, but rather depends on the data rate because of capacitive sampling of the input. That sampling can interact with the input voltage source and reduce the real resolution and accuracy. Anyway, the input impedance is not nearly high enough to take full advantage of the accuracy and resolution of the A/D converter when used with higher impedance voltage sources.
That means a high-impedance buffer must be placed in front of the A/D converter. That buffer will introduce small offset and gain errors, so the the built-in self-calibration circuitry can no longer do the whole calibration job. We'll come back to that. The high-impedance buffer section needs to have excellent common mode rejection. Let's take a minute to explain what that means. A single-ended A/D input measures voltage in comparison to a ground potential, theoretically zero volts. The problem with that is that no two points can be relied on to be at the exact same potential. A 24-bit A/D converter with a +/-5 voltage range, like the Model 201, resolves less than 1 microvolt. It doesn't take much resistance or current flow to create a 1 uV voltage drop across a wire. There are also inevitable AC voltages present, but we talk about AC noise in other places, so will stick to the DC effects here. In order for the last few bits of a 24 bit A/D to have real meaning, you need to make a differential measurement, instead of relying on a hypothetical zero-volt ground. As you might guess, a differential measurement responds to the difference between the plus input and the minus input. For an ideal differential input, you could connect both inputs to a +/-5 volt sine wave and the reading should stay zero, regardless.
In practice, the gain from the minus side will not perfectly match the gain from the plus side, so some small remnant of the sine wave of common mode input voltage would show in your readings. We need that remnant to be vanishingly small in order to be able to ignore common mode effects. In the Model 201, the common mode rejection is minimized with a hardware trim. That leaves the offset and gain errors that occur in the added high impedance differential buffer section. You can try to trim out these errors in the hardware, but the process is necessarily imperfect, especially when you measure over a large temperature range. Also, offset and gain trims interact, and trimpots can change resistance over time and as the result of vibration. Best is to include the offset and gain errors in the self-calibration process. Again, that means the built-in self-calibration cannot do the whole job, if you are going to maintain the maximum DC accuracy.
A multiplexer is placed in front of the new amplifier so that calibration signals can run through the input buffer to the A/D. In the case of the Model 201, there is an 8-channel differential multiplexer with six channels for general-purpose input and two channels dedicated for calibration. All six available inputs have series protection resistors. These do not introduce noticeable error because of the extra-high input impedance buffers that follow. Then, the entire system is calibrated by both the internal self-calibration, and by the external signals, as managed by the external microcontroller.
To sum it up, the front end of the circuitry includes a protected multiplexer, a high-impedance differential amplifier stage, and calibration signals. The output of all that is fed to the A/D chip, under microprocessor control. Actually, there are two more circuit blocks between the input section and the A/D chip.
Overvoltage protection
Over enough time, all sorts of mishaps at the analog inputs are bound to occur. The easy way to protect the A/D chip is with clamp diodes to the supply rails. Clamp diodes can leak enough current to add measurable error to a high impedance input. Also, clamp diodes turn on incrementally over a range of voltage and may not save the A/D chip when the transient hits. A better clamp, for more reliability and best accuracy, requires a comparator and an analog switch to guarantee proper protective clamping.
Programmable filter
Delta sigma converters like the AD7712 are extremely effective at rejecting most frequencies of input noise. Still, for any given data rate, there are certain frequencies that elude the digital filtration. The solution to that problem is an analog low pass pre-filter that will remove the unwanted frequencies. Active filters introduce DC errors, so we avoid them. A simple one-pole RC filter will do the low pass job, but because the Model 201 is digitally programmable over a wide range of data rates, different filter constants are appropriate for different circumstances. So, the Model 201 has three programmable analog filter time constants. That added functionality involves adding a high-quality polypropylene filter capacitor and a mechanism to switch in different series resistors.
Power input and power supplies
The power input is given special treatment. First, we protect against reversed connections, then against overvoltage. Third, we pre-regulate at 24 volts, for any case where the input voltage is higher than that. Then, a 5 volt standby supply is produced. The standby supply is always on to keep the microcontroller active and checking for serial commands. If there is serial activity, the microcontroller powers up the rest of the power supply chain. The preconditioned input voltage of 14 to 24 volts is regulated at 12 volts. From that, a charge pump produces a -12 volt supply. Those +/- 12 volt supplies are regulated, but not precisely regulated. The +/- 12 volts is re-regulated to +/-6.2 volts to power the sensitive analog circuitry. An analog 5 volt supply is also derived, using a precision 5 volt reference that runs from the +12 volts. There are two more reference supplies needed by the A/D converter in order to take full advantage of its dynamic range. Those are at +/- 2.5 volts. Finally, a precision 5 volt reference output is provided for off-board circuitry.
It adds up to 12 power supply voltages total. In addition, various connections to the supplies are decoupled from each other with small-value series resistors and capacitor filters. A typical decoupling network might be a 10 ohm series resistor with a 10 uF and a 0.1 uF shunt capacitor. Decoupling networks eliminate unwanted interactions that can propagate via the power supplies.
To allow battery powered applications, total current drain must be kept low. The Model 201 draws just about 1.5 mA in sleep mode, and about 17 mA during normal operation.
Optical isolation
There are a few more power supply components on the Model 201 board, but the power for them comes from the host computer serial port. The RS232 interface provides plus and minus power to a chip which drives, and is driven by, the optocouplers for the serial communication. The reasons for needing this additional level of isolation are explained in detail in other posts on this discussion site. For now, just remember that isolation breaks ground loops and protects against potentially damaging fault currents.
The remaining circuit blocks on the Model 201 board are for digital input and output, and for expansion. There is also another layer of optical isolation for the four expansion outputs, A through D.
Overview of the microcontroller function:
Back in the '80s affordable microcontrollers did not include peripherals like serial interfaces. The PIC processor selected for the Model 201 was fast and efficient for its day, but unadorned with extra features. We built a stripped-down real-time operating system that prioritized serial communications and getting the analog data out of the A/D chip and into a small buffer. Digital input and output, plus housekeeping were handled as lower priority tasks. For data logging applications, the microcontroller can keep time and send back a pre-defined data set as fast as 1000 times per second or as slowly as once a day. Alternatively, it can passively respond to commands from the host computer.
The Model 201 is still an active product 35 years after its introduction, so it has proven itself in the marketplace. If you need just a little more than 16-bits of real resolution, you can get away with lot by starting with a 24-bit delta sigma A/D converter chip. But, if you want 22 or 23 bits of real, usable, reliable resolution, you need to do everything exactly right.
Tom Lawson
February 2022