Digital vs. Mechanical Potentiometers: Design Considerations To Maximize System Performance

As with all components there are some factors to consider when choosing the correct component for your application. The ranking of how important each specification is will depend on the final use and other system considerations listed in Table 1.

Some applications that digital potentiometers are commonly used in are:

·     Attenuators for DC and AC signals

·     Varying the gain of an Op-Amp

Using Digital Potentiometers As Attenuators

A digital potentiometer can be used to emulate a simple low resolution digital-to-analog converter (DAC).  Fig. 2 shows this setup as well as some terminology that is frequently seen. The, the resistance between terminals A and B.  The transfer function is also noted in Fig. 2

Where:

R_AB = End resistance

R_WB = Resistance between wiper and B terminal

V_REF = Reference voltage

The best way to understand these considerations is to take a look at how they affect the choice of digital potentiometer in a specific application and so we will now look a two important use cases for digital potentiometers in more detail.

For this setup, there are three key parameters to note while choosing a digital potentiometer:

·     Voltage Supply Range

·     Digital potentiometer resolution

·     Digital potentiometer linearity

  Voltage Supply¹ and Resolution² are important considerations as these specifications cover the range of inputs the digital potentiometer can pass and number of distinct resistance levels that can be achieved. The linearity of a digital potentiometer is specified in a similar manner to DAC’s using INL (Integral Nonlinearity) and DNL (Digital Nonlinearity).  INL refers to the maximum deviation of a real digital potentiometer from an ideal straight line drawn from zero-scale to full-scale.  DNL refers to the difference between the output and ideal transfer function for successive codes.

For AC applications, the same parameters as DC power supplies apply also (Voltage Supply Range, Resolution and Linearity). Total Harmonic Distortion (THD) and Bandwidth are key factors that should also be considered.

Digital potentiometers are very useful in varying the gain of an op-amp.  The gain ratio of R_b/R_a can be set and varied precisely using the digital potentiometer.  Applications that utilise gain control include volume control, sensor calibration and contrast/brightness in LCD screens.   However, there are a number of digital potentiometer characteristics that must be considered during configuration.

When a digital potentiometer is used in potentiometer mode, one must be aware of the transfer function of the digital potentiometer as the resistance increases from zero-scale to full-scale.  As the resistance between R_AW increases, R_BW decreases, this creates a logarithmic transfer function Logarithmic transfer functions are more suited to the human ear and eye response (Fig. 3(a)).

If the application requires a linear response, the digital potentiometer can be linearized either by using the digital potentiometer in rheostat mode (Fig. 3(b)), a Vernier DAC configuration (Fig. 3(c)) or through the “linear gain setting mode”, a feature exclusive to members of the ADI digiPOT+ family such as the AD5144.(Fig. 3(d)).

Rheostat Mode With a Discrete R’esistor:

Using the digital potentiometer in rheostat mode and placing it in series with a discrete resistor, the output can be linearized (Fig. 3(b)). This is a simple design however there are design considerations that need to be taken into account to maintain system accuracy.

Both mechanical and digital potentiometers have a tolerance on the resistance for different reasons. For mechanical potentiometers, the tolerance can vary due to the difficulty of achieving repeatable values. For digital potentiometers, there is tolerance due to the manufacturing process but it has values that are far more repeatable compared to a mechanical potentiometer.

A discrete surface mount resistor may have an offset as small as 1%, but some digital potentiometers can have an end-to-end resistance tolerance of up to 20%.  It is this mismatch that can lead to loss in resolution and can be a significant problem, particularly in open loop applications where monitoring is not practical to compensate for the error. Where monitoring is possible the built-in flexibility of digital potentiometers allows a simple calibration routine to adjust the wiper position of the digital potentiometer and adjust for any offsets.

Analog Devices’ portfolio of digital potentiometers are specified with varying tolerances of 20% down to 1% to match the most stringent precision and accuracy needs.  Some digital potentiometers, such as the AD5258/AD5259, factory test the error tolerance and store the result in user accessible memory to allow resistor matching at production.

Linear Gain Setting Mode

The final method is the Linear Gain Setting Mode exclusive to ADI’s digiPOT+ portfolio. Fig. 3(d) shows how the patented architecture implemented allows for independent programming of the value for each string R_AW and R_WB. Using this mode allows for a linear output by fixing the output of one string (R_WB) and setting the other string (R_AW). This operation is similar to using the rheostat mode digital potentiometer with discrete resistor however the overall tolerance error is less than 1% without any extra parallel or series resistance combination.

This is due to the error in the resistances is common in both resistor string arrays and can be disregarded. Fig. 4 shows the mismatch error between the two resistances is small at higher codes. The mismatching does increase above ±1% at codes lower than quarter-scale but this is due to the error added by the effect of the internal CMOS switches resistance which cannot be ignored.

Importance of memory

When using digital potentiometers to set levels in circuits or calibrate sensors and gain settings the power up state of the digital potentiometer is important to ensure accurate and speedy configuration. Digital potentiometers are available in many options to ensure the device powers up in the users preferred state. There are two categories of digital potentiometer:

·     Volatile – Parts have an on-chip memory element which stores the user chosen wiper position to be configured on power up.

·     Non-volatile – Parts do not have a programmable memory, instead the part powers up the wiper position at zero scale, mid-scale or full scale depending on the part configuration. See each products data sheet for full details.

Within the non-volatile digital potentiometer classification there are further options

·     EEPROM

·     One Time Programmable (OTP)

·     Multiple Time Programmable (MTP)

The wide variety of memory options allow the digital potentiometer choice to be tailored to the particular system. For example, for systems that require constant adjustment, volatile digital potentiometers can be used. For systems that only require a calibration on factory testing, an OTP potentiometer can be used. EEPROM digital potentiometers can be used to retain the last wiper position so that on power up the digital potentiometer will return to the last state and can continue to be adjusted as needed after power up.