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Reduce redesign risk with correct ADC analogue input

Posted: 17 Nov 2015 ?? ?Print Version ?Bookmark and Share

Keywords:SAR ADCs? data converter? unipolar? bipolar? power consumption?

Successive approximation register analogue to digital converters, more commonly known as SAR ADCs, are a versatile class of analogue to digital converters that produce a digital discrete time representation of a continuous analogue waveform. From a high level, the SAR ADC is a versatile, easy-to-use, completely asynchronous data converter. Still, there are some choices that need to be made when deciding which converter is to be used for a particular application. Here specifically we will focus on the analogue input signal types available to the signal chain designer. It should be noted however that even though the focus here is on SAR ADCs, the input types are universal to all ADC architectures.

Depending upon the source type or the overall goals of the circuit under consideration, certain design decisions and trade-offs will need to be made. The simplest solution would be to match the ADC input type to the signal source output configuration. However, the source signal may require conditioning that changes the signal type or there could be cost, power or area considerations that influence analogue input type decisions. Let's explore the different available analogue input types.

The simplest analogue input type is the single-ended input. In this case, there will be a single input pin with no direct return or sense path back to the signal source. The conversion result will be generated with respect to the ground pin of the ADC. Depending upon the specific device, the input may either be unipolar or bipolar. In the single-ended architecture, simplicity is its merit. Only one trace is required to get a signal from its source to the ADC. This can reduce the complexity of a system while lowering the power dissipation of the overall signal chain. There is potentially a trade-off for this simplicity. A single-ended setup does not reject DC offsets that are present within the signal chain.

The single-ended system relies upon making measurements relative to a current carrying ground plane and voltage differences between the source ground and ADC ground will appear in the conversion results. As well, the setup is more susceptible to coupled noise. Therefore, the signal source and the ADC should be kept close together to mitigate these effects. If the SAR ADC is a unipolar single-ended configuration, the allowable signal swing is between ground and positive full-scale, which is typically set by the ADC reference input. A visual representation of a single ended unipolar input can be seen in figure 1.

Figure 1: Unipolar Single-Ended.

If the SAR ADC is a bipolar single ended configuration, the allowable signal swing is between positive full-scale and negative full-scale relative to ground. Again, full-scale is typically set by the ADC reference input. A visual representation of a single ended bipolar input can be seen in figure 2.

Figure 2: Bipolar Single-Ended.

If the need arises to sense the signal ground or decouple the relative measurement result from the current carrying ground plane, the signal chain designer may want to consider a move to a pseudo-differential input structure. A pseudo-differential device is essentially a single ended ADC with a ground sense. The device is performing a differential measurement, but the differential that is sensed is a single-ended input signal measured with respect to the input signal ground level. The single-ended input is driven onto the positive input (IN+) of the ADC and the input ground level is driven onto the negative input (IN?) of the ADC.

One thing to note is the signal chain designer must pay attention to the analogue input range of the negative input. In some cases, the negative input pin has a limited input range compared to the positive input. In these cases, the positive input can freely swing through the allowable input voltage range, while the negative input to the ADC may be restricted to some smaller voltage range around ADC ground. The allowable input ranges for each ADC input can be found in the data sheet. Look for a specification named Absolute Input Voltage as seen in figure 3.

Figure 3: Absolute Input Voltage Example.

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