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While reading about Pseudo-Differential Inputs for ADCs, I came across the document below from LT. Link : http://cds.linear.com/docs/en/product-selector-card/2PB_sarinputtypesfb.pdf

Below are copied lines from the PDF:

"A pseudo-differential unipolar ADC digitizes the differential analog input voltage (IN+ – IN–) over a span of 0V to VFS. In this range, a single-ended unipolar input signal, driven on the IN+ pin, is measured with respect to the signal ground reference level, driven on the IN – pin. The IN+ pin is allowed to swing from GND to VFS, while the IN– pin is restricted to around GND ± 100mV (Figure 2a).

A pseudo-differential bipolar ADC digitizes the differential analog input voltage (IN+ – IN–) over a span of ±VFS/2. In this range, a single-ended bipolar input signal, driven on the IN+ pin, is measured with respect to the signal mid-scale reference level, driven on the IN– pin. The IN+ pin is allowed to swing from GND to VFS, while the IN– pin is restricted to around VFS/2 ± 100mV (Figure 2b)."

Now the questions is, how do we make a choice between Pseudo-Differential Unipolar and Pseudo-Differential Bipolar. The choice should be based on the property and characteristics of the Input signal. For example, the input signal is a single ended sin function with range from 0 to 3V and a DC bias of 1.5V. Should we connect the IN– to 1.5V or to GND?

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2 Answers 2

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Pseudo differential takes a full positive swing on the +IN input whether it is unipolar or bipolar. If it is bipolar the -IN needs to be fed the nominal dc offset of the signal fed to +IN.

This is an advantage on analogue systems using a mid-rail generator (such as a voltage reference and op-amp buffer). If the voltage reference moves up or down a little, it doesn't affect the ADC code value produced because that offset movement will affect both input signals together.

For a unipolar input where -IN is 0V and a mid-rail generator is used, if the mid-rail generator shifts a little, then that is reflected as a dc error in the converted result.

I tend to use unipolar configurations (even when a mid-rail generator is used) because there appears to me more choice in ADCs AND I always choose really good references PLUS, in a lot of signal processing I do, I don't care about (say) a 10mV offset end-to-end.

For example, the input signal is a single ended sin function with range from 0 to 3V and a DC bias of 1.5V. Should we connect the IN– to 1.5V or to GND?

  • If the dc bias of 1.5 volts and very stable it doesn't matter which you use
  • If you don't care too much about small dc offset variations then use either
  • If you do care about offset variations then use bipolar pseudo differential
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You can choose what you want !

Either IN- to 1.5 V (the average of the signal on IN+) or simply GND. But note that this choice restricts what ADC chip you can use, see the table on the last page. Note that some ADCs can handle both PDU and PDB.

In practice it also depends on:

  • How will the input signal excite the ADC to full range ? (don't want to lose dynamic range)
  • Does my input signal have a predictable average value (DC + sine: yes; some temperature sensor's signal: no)
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