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How to assist ADCs with external analog drivers

In my previous Discrete Signals blog, I asked a question. Is using an analog-to-digital converter (ADC) as simple as tying a sensor output to its analog input and starting to take readings? Astute readers saw I only answered part of that question—ground is often the culprit when ADC readings go haywire, but it’s not the only one.

Driving the analog input also deserves some attention from makers. Three situations call for assisting ADCs with external analog drivers. Let’s consider cases of small signals, noisy signals, and faster signals.

  1. Using as much range as possible

Many analog sensors produce a very small voltage swing from minimum to maximum readings. Plus, there may be a big difference in the theoretical swing versus the actual swing seen during operation. Engineers like to size things for worst case. It can be a bad habit when using ADCs without outside help in scaling signals properly.

A simple example is a thermometer designed for home use. Readings ranging from hypothermia at 35°C to hyperpyrexia at 41°C likely use only part of the measurement capability of the sensor in the device. Another example is an accelerometer; there’s no need for a ±30 g part when expectations are ±6 g.

What happens when readings use only a fraction of the range? Think about it. If the upper eight bits never change during ADC operation, congratulations, we’ve turned a 12-bit converter into a 4-bit converter. Drop the resolution, and nuances in signals disappear.

This implies a three-step process for makers designing sensor systems:

  • Choose a sensor with enough range plus some margin for the swing expected during normal operation.
  • Amplify and shift the sensor signal to take half the ADC input range if a midpoint glitch is a concern.
  • Select an ADC with appropriate resolution to space out readings in that half range.
  1. Dealing with a bit of noise, or a bit more

External amplification also helps in another way. Stronger analog signals are more noise resistant, and digital averaging ADC readings can help smooth out white noise. A bit of boost might raise signal-to-noise ratio (SNR) enough for a single-ended analog subsystem to produce good results.

We touched on the idea of single-ended versus differential signaling last time. A single-ended approach uses only one wire to carry an analog signal, relying on a system-wide ground for its return path. A differential approach uses two wires, a plus and minus, impressing a voltage difference independent of ground. Twisted pair wire can help differential signaling stay quiet, providing a cancelling effect for noise on both wires.

A simplified illustration highlights single-ended versus differential ADC signaling. Source: Stratiset

Implied here is an upgrade to an ADC using differential inputs. Most low-cost ADCs, including ones found on microcontrollers, are single-ended. Differential inputs are a common feature of precision ADCs at higher sample rates, and it may be worth getting the number of bits expected.

  1. Higher sample rates

Despite our best efforts to simplify things, sometimes the physics become unavoidable. In many ADC architectures, their analog inputs look capacitive, making things a bit more complex. For example, some ADCs use a switched-capacitor scheme in a sample-and-hold to stabilize the signal for capture. Too big a signal change, too little current drive, and not enough charging time, and the sample never reaches the level of the incoming signal.

Overcoming the capacitive surge calls for ADC drivers that can pump enough current quickly. Slew rate measures how fast a driver creates an output voltage change responding to its input. A high slew rate ADC driver also needs more bandwidth, anywhere from five to 10 times the sample rate of the ADC driven. At very high sample rates in radar and 5G designs, transmission line theory and impedance matching come into play, which is beyond the scope for most makers.

Easier selection of parts for the job

We’ve made a lot of progress in ADC science, which is maturing to the point where optimized parts target certain applications. Most ADC vendors also have selections of ADC drivers created for specific jobs. That allows engineers to handle differential signaling, slew rate, bandwidth, common-mode noise rejection, and more.

Assisting ADCs with external analog drivers sets the right scale for pulling signals out of noise. Combined with a sensor spanning the expected range of physical input, a driver gives the ADC use of more of its digital range. As speeds increase, an ADC driver can help makers work in audio, radio, or video applications.

After spending a decade in missile guidance systems at General Dynamics, Don Dingee became an evangelist for VMEbus and single-board computer technology at Motorola. He writes about sensors, ADCs/DACs, and signal processing for Planet Analog.

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