Digitalized measurements of analog signals require an ADC to quantize the level of a time-domain signal within some required frequency range. Selecting an ADC for a precision measurement application is not the whole story of using ADCs. In fact, there is an important point to consider when selecting an ADC: the voltage reference. In an ADC circuit, the reference is used as a benchmark to compare signal levels, and the stability of your reference voltage source will determine the accuracy of your measurements.
Precision references are not always required for use in ADCs, and it’s likely a novice designer will just connect the reference pin on an ADC to their power supply. This might be fine in very simple ADC applications, such as systems with high dynamic range (and ENOB), high SNR, low resolution, and stable temperature. In the other case, where there is potential for quantization errors in high resolution sensing systems for low SNR signals, the ADC referencing method becomes very important.
In this article, we’ll look at two common methods to supply a reference voltage to an ADC: a precision reference component (available in standard IC packaging) and a ratiometric connection to the system power source.
What Are ADC References?
All ADCs operate by comparing a sensed input voltage with a reference voltage. In the simplest topology, this is done with an op-amp circuit that continuously compares the input to a scaled value of the reference voltage. The reference voltage determines the highest signal level that the ADC can convert, and all quantized digital outputs from an ADC will be some ratio of this input reference voltage. Therefore, the accuracy of the reference voltage level is of great concern when precision measurements are needed.
Some ADCs have an internal reference, or they can be configured to use either an internal or external reference. There are some advantages to using an external reference for an ADC, most principally centering around accuracy. If you opt for an external reference that can guarantee greater stability against power and temperature changes, the two standard approaches are to use a precision voltage reference or a ratiometric reference.
Precision Voltage Reference IC
The integrated circuits are designed to convert an input (possibly noisy) voltage source into a highly stable voltage source at a specific value. The output is then connected to the ADC reference voltage pin. In the PCB layout, the best practice is to route the output from the precision voltage reference to the input pin as a small rail, ideally shielded with ground on adjacent layers and on the same layer.
In this topology, the sensor excitation/readout subcircuit does not need to have the same power source as the reference. However, these could be the same power source, both being independently stepped down from the main power source.
These references are intended to have very low drift and be relatively noise-immune, particularly to noise on the input power rail. In other words, the voltage they supply should not change while the system is in operation. Therefore, they can function as a much more accurate voltage reference than an internal reference when one is needed.
In this type of ADC reference, the same power source used to excite your sensor or analog signal source is also used to set the voltage reference for the ADC. An example of this connection is shown below. In this case, if the input voltage is larger than the reference voltage required by the ADC, you can pass the reference voltage through a resistive divider (V-div). This will reduce the input voltage by a specific ratio while also ensuring there is no phasing in any low-frequency fluctuations in the reference voltage.
Ratiometric ADC reference topology.
Ratiometric references are only appropriate in situations where the signal being measured is supplied by the sensor excitation voltage source. Note that, in the above topology, you could use a precision voltage reference if you wanted, a ratiometric reference is not required, but a ratiometric approach does give some benefits.
The main benefit of a ratiometric ADC reference compared to the use of a precision voltage reference is the smaller system size. As long as the power source is stable and precision resistors (0.1% tolerance) are used in the voltage divider, the ADC reference can be made very stable.
Next, if there is any fluctuation in the power system, such as brief voltage droop, the reference will drop by a proportional amount (assuming the sensor excitation/readout subcircuit is linear). As a result, the measurement system still has the same dynamic range. The danger here is that the input voltage droops, but the noise floor in the excitation/readout subcircuit stays the same. If this happens, there are two possible results:
- If the sampled signal is already at the high end of the ADC input range, you might then saturate the ADC’s input and get a false maximum reading. This can happen if the signal excitation/readout power does not change during the power droop.
- If the sampled signal also experiences a power droop, then the signal’s SNR value would decrease when the voltage droop occurs, which increases the chances of quantization errors.
Other Mixed-Signal PCB Design Tips With Sensors
Mixed-signal systems will often use ADCs for sensor data capture and precision measurements, and these sensor systems will often operate at the lower end of the frequency range. These are typically in the high kHz range up to the MHz range so as not to be sensitive to radio communication protocols (starting from sub-GHz and up to 5-6 GHz for most wireless systems).
- Make sure you understand the return paths for the digital interface on your ADC
- Use a solid ground plane; do not split up the GND net into regions unless your sensor interface has low SNR and is at low frequency
- Implement best power system design practices for the digital interface
- If needed, slow down the digital interface on the ADC (usually SPI or I2C) with an RC circuit (for inputs) or series resistor (for outputs)
When you need to place and route ADC references in your PCB layout, make sure you use Allegro PCB Designer, the industry’s best PCB design and analysis software from Cadence. Allegro users can access a complete set of schematic capture features, mixed-signal simulations in PSpice, and powerful CAD features, and much more.
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