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How Many of Those 24 Bits are Real? - Tom - 06-14-2021
If your purpose is to extract the most resolution and accuracy and reliability out of a 24-bit A/D converter chip, it takes more than wiring up a few connections. Here we look at the Lawson Labs Model 201, and analyze the various design considerations that went into it. The Model 201 is a worthy choice for this exercise because it is still going strong, 29 years after its conception. Lawson Labs has a reputation for reducing a task to its simplest terms. You can hang an A/D chip on a microprocessor with just a few additional components. Why does the Model 201 have so many parts?
Analog inputs
24 bit A/D chips like the AD7712 have built in self-calibration circuitry, which we employ. However, the common mode rejection is not nearly good enough to take full advantage of the accuracy and resolution of the A/D converter, so we add an input amplifier with excellent common mode rejection. That amplifier adds its own offset and gain errors, so an additional active calibration mechanism needs to be provided in order to maintain the DC accuracy.
A multiplexer must be placed in front of the new amplifier in order to to be able to run calibration signals through it to the A/D. In the case of the Model 201, there is an 8-channel differential multiplexer with six channels for input and two channels for calibration. All six available inputs have series protection resistors. Those resistors mean we require higher input impedance for the amplifier in order to maintain full accuracy. That high impedance is desirable for other reasons, as well. So, more than a little additional circuitry has been added to the Model 201 to obtain excellent common mode rejection.
Power input and power supplies
Most of the “extra” parts on the Model 201 are related to power supply regulation and decoupling. The Model 201 is designed for maximum flexibility of power input. It can be run from a 12V battery, a 48 volt DC supply, or from a wall adapter. For battery operation, power consumption must be minimized. Lower power means less self-heating, which is also an advantage for DC accuracy. First the power input is protected against reverse connection. Then, it is protected against overvoltage. Then, a 5 volt standby supply is generated. This supply is always on to keep the microcontroller alive and checking for wake-up commands. Next, the raw power input is regulated at 12 volts. From that, a charge pump produces a -12 volt supply. The +/- 12volt supplies are thus regulated, but not precisely regulated. The +/- 12 volt supplies are switched on and off by the microcontroller. They do not operate when the Model 201 is asleep, saving power. When awake, to power the sensitive analog circuitry the +/- 12 volts is re-regulated to +/-7.5 volts. An precise analog 5 volt supply is also derived, using a precision 5 volt reference that runs from the +12 volt supply. 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.
Lets count - rectified filtered raw input, 5 volt standby, + 12, -12, + 7.5, -7.5, two 5 volt references, analog 5 volt supply, digital 5 v supply, +2.5v, and -2.5. It adds up to 12 power supplies 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. Just one decoupling network per power supply would add 36 components. In fact, there are more decoupling components than that.
How does one determine how many decoupling networks to include? It is largely a matter of trial and error. My process was to exercise the A/D under the most demanding dynamic conditions and looking for any interactions. One good test is to apply a square wave to one channel and look for crosstalk on other channels. Another is to add noise on the raw power input and to look for additional noise anywhere else in the circuitry. Loading the digital outputs, and turning them rapidly on and off together should not affect the data. If it does, there is more decoupling work to do to isolate the analog and digital sides of the interface.
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 provides plus and minus power to an interface chip which drives, and is driven by, the optocouplers for the serial interface. 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.
Back to the analog side - 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. That circuitry adds another handful of components to the Model 201.
Programmable filter
Delta sigma converters are wonderfully 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 can 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 filter capacitor and a half dozen other parts.
The remaining components on the Model 201 board are for digital input and output, and for expansion. There is also optical isolation for the four expansion outputs, A through D. Count two crystals for clocks and one decoding chip and one latch to multiplex pins on the microcontroller itself.
That is everything. All those extra parts, you see, are there for a reason. If you leave any of them off, there is a price to be paid. Maybe that is why the Model 201 is still an active, desirable product 35 years later. If you just need a little more than 16-bits of real resolution, you can get away with lot, when 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
June 2021