Mastering Electronics Design

Articles on Internet and books show how to calculate the Least Significant Bit (LSB), but they take into consideration either the voltage reference (Vref) or the full scale (FS) of the ADC or DAC.  Many times this leads to confusion, as a few messages I received from my readers show.  Therefore, this article shows both ways of defining the LSB, so that people will have a clear understanding how to treat an ADC’s (Analog-to-Digital-Converter) or DAC’s (Digital-to-Analog-Converter) LSB.

What is an LSB? The LSB is the smallest level that an ADC can convert, or is the smallest increment a DAC outputs.  Both converters are used at the boundaries between the analog and digital realms, making it possible for the analog circuits to talk to the digital ones and backwards.

To define an LSB, let’s not look, for the moment, at articles or text books.   One of the recommendations I make is this: “When in doubt, read the manufacturer’s data sheet.” So, let us look in an ADC data sheet.  The manufacturer writes in the data sheet all the information one needs about that particular component.  Sure, nothing is perfect in this world, some info we need might not be there, but in the case of the LSB, I still have yet to see an ADC or DAC data sheet without the LSB definition in some form or another.

The ADC needs a voltage reference to convert an analog signal into a digital word.  Depending on the number of bits it has, the ADC divides the voltage reference in small levels called counts.  For example, if this is an 8-bit ADC, the counts will look like those in Figure 1.  In an 8-bit ADC there are 2⁸ = 256 counts.

Figure 1

One count is 1 LSB, and is defined as follows:

(1)

where N is the ADC’s or DAC’s number of bits.

For ADCs that have a differential voltage reference, the LSB is

(2)

where Vref(+) and Vref(-) are the non-inverting and the inverting inputs of the differential voltage reference respectively.

The ADC outputs a digital word that shows how many counts are in its input voltage level.  As the ADC counts the input level, it never reaches the voltage reference.  Its full scale (FS) is calculated with the following formula:

(3)

After replacing the LSB in equation (3), the ADC full-scale results as in equation (4).

(4)

Figure 2 shows the ADC counts and the corresponding hex code.

Figure 2

In our 8-bit ADC example, if the voltage reference is Vref = 5V, then the LSB and FS are:

(5)

As you can see, and ADC can never reach its Vref but, as the number of bits is higher, it gets very close to its reference voltage.  The same can be said about a DAC.

Moreover, from equation (1), we can write the mathematical relation between Vref and LSB as follows:

(6)

If we replace Vref in equation (3), and after calculations, we can write the definition of the LSB as a function of the ADC’s full-scale, as in equation (7).

(7)

This is the trouble, as the LSB has two definitions, equations (1) and (7).  Both of them are valid, and some authors are ambiguous or confused about them.   I have seen articles in which Vref is considered the component full-scale, which is the premise that generates subsequent wrong definitions.

Therefore, all one needs to remember about the LSB is its definition, as follows:

(8)

>>> <<<

Here are some data sheet examples:

AD7823, 8-Bit ADC, manufacturer Analog Devices

If Vref = 2.5 V then
1LSB = 9.766 mV,
FS = Vref – 1 LSB = 2.490234 V
If Vin = FS = 2.490234 V, then ADC Hex Code = FF, and
If Vin = 0 V, then ADC Hex Code = 00.

If Vref = 5 V then
1LSB = 19.531 mV,
FS = Vref – 1 LSB = 4.980469 V
If Vin = FS = 4.980469 V, then ADC Hex Code = FF, and
If Vin = 0 V, then ADC Hex Code = 00.

AD7685, 16-Bit ADC, manufacturer Analog Devices

If Vref = 2.5 V then
1LSB = 38.1 uV,
FS = Vref – 1 LSB = 2.499962 V
If Vin = FS = 2.499962 V, then ADC Hex Code = FFFF, and
If Vin = 0 V, then ADC Hex Code = 0000.

If Vref = 5 V then
1LSB = 76.3 uV,
FS = Vref – 1 LSB = 4.999924 V
If Vin = FS = 4.999924 V, then ADC Hex Code = FFFF, and
If Vin = 0 V, then ADC Hex Code = 0000.

DAC8830, 16-Bit DAC, manufacturer Texas Instruments

If Vref = 2.5 V then
1LSB = 38.1 uV,
FS = Vref – 1 LSB = 2.499962 V
If DAC Hex Code = FFFF, then Vout = FS = 2.499962 V, and
If DAC Hex Code = 0000, then Vout = 0 V.

If Vref = 5 V then
1LSB = 76.3 uV,
FS = Vref – 1 LSB = 4.999924 V
If DAC Hex Code = FFFF, then Vout = FS = 4.999924 V, and
If DAC Hex Code = 0000, then Vout = 0 V.

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