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Circuit Surgery Regular clinic by Ian Bell Electronically controlled resistance – Part 4 of steps (resistance values or wiper positions, often referred A A to as ‘taps’) is usually a power Control input Control input of two related to the number (R value) (wiper position) Wiper R R of bits in the digital control circuit, with the range of steps being typically from 32 (5-bit) B B to 1024 (10-bit) but fewer steps Single resistor (rheostat) Potentiometer (4 and 16) and 100-step devices are also available. Fig.1. Basic digipot types: representing the electroThe stepped nature of digipot mechanical rheostat and potentiometer. adjustment is different from the theoretically continuous variation available from a film-based mechanical or the previous three months potentiometer or JFET electronically controlled resistors. Real mechanical we have been discussing electronipotentiometers are not perfect so do cally controlled resistance, focusing not have an infinite effective number mainly on the use of JFETs (junction fieldof steps. Their effective resolution and effect transistors) as voltage-controlled repeatability (the accuracy with which resistors. This series was instigated by the same value can be set after moving a post on the EEWeb forum about elecaway) may be about 0.5% to 1%. Multitronically controlling resistance, but not turn mechanical potentiometers provide specifically about JFETs. Although JFETs better resolution (finer adjustment) at (and sometimes other FETs) are comhigher cost. Wirewound mechanical monly used as electronically controlled potentiometer have lower resolutions resistors, and have significant historical because the wire turns effectively create significance, they are not the most widely mechanical steps in the resistance value. used approach in modern systems. So, this The resolution of an N-step digipot is month we will look at ‘digipots’ – digital potentiometers – which are integrated circuits providing a similar function to mechanical potentiometers and trimmers. Digipots are available in both rheostat and potentiometer versions (see Control logic Fig.1). Potentiometer versions can usually also be used in rheostat mode by using the Control signals resistance between the wiper or standard bus and one of the ends (A or B). Digital Resistance Switch The resistance (rheostat) or interface register logic wiper position (potentiometer) is controlled with a digital i n t e r f a c e . Ty p i c a l l y , digipots are controlled by a microcontroller, often via a serial bus such as SPI or I2C, but simpler interfaces which allow the resistance or position to be stepped up or down, as well as parallel digital inputs are also available. The number Fig.2. Circuit concept for implementation of a digipot. 100/N %. For example, with an 8-bit control word this is 100/28 = 100/256 = 0.4%. 54 Practical Electronics | December | 2022 F Digipot circuit Before discussing digipot devices or parameters further it is useful to look at a basic digipot circuit, as this provides insights to their characteristics. Fig.2 shows a digipot implementation concept diagram – it is not intended to represent a specific device. The potentiometer comprises a set of resistors connected in series between A and B – this arrangement is often referred to as a resistor ladder. The total resistance between A and B is fixed and is referred to as the end-to-end resistance, or simply the resistance of the digipot. This resistance varies widely for available devices – for example, from 1kΩ to 1MΩ – but devices with 5kΩ to 100kΩ resistance are more common. Often, a given IC will be available in a few different resistance values. The wiper is implemented using a set of electronic switches. One of the switches will be on to determine the wiper position, all the other switches are off. The A, B and wiper (W) connections correspond with the standard potentiometer symbol shown in Fig.1. The wiper switches connect to all A SN RN–1 SN RN–2 SN–1 SN–2 SN-1 SN–2 RN–3 Wiper (W) S1 S0 R1 R0 S1 S0 B the points between the resistors in the ladder, so they are often referred to as ‘taps’. The switches are typically implemented using MOSFETs (transmission gates using an N and P transistor in parallel). Which switch is on is determined by the control logic. In the circuit in Fig.2 the resistance register holds a binary number controlling the wiper position. This is decoded by the switch logic to determine which single switch is on. The register value is set to the required wiper position value by the user via the interface logic. The interface can take a number of forms, as already noted, and standard serial buses and up/down controls are common. Digipot ICs may have multiple potentiometers on a chip (typically 2, 4 or 6). Resolution and variations The number of resistors between A and B (Fig.2) determines the resolution of the potentiometer and hence the number of bits required in the resistance register. If all the resistors have the same value, called the step resistance (RS), then the resistance from the A or B to the wiper (RAW or RBW) will have a linear relationship with the resistance register value – this is called a ‘linear taper’. It is also possible to scale the resistor values so that the resistance from the A or B terminals to the wiper varies logarithmically with the resistance register value (called a ‘log taper’). The linear and log taper options directly correspond with linear and logarithmic mechanical potentiometers. Logarithmic potentiometers are frequently used in audio equipment as volume controls because of the logarithmic response of the human ear to the loudness of sound. Using a volume / gain control with a logarithmic taper digital pot will change the signal level in constant decibel steps (eg, 1 dB per step). There are variations possible in the number of resistors and wiper taps for digipots with a similar resolution. If we have a 7-bit resistance register then there are 27 = 128 possible values, which means that 128 switches can be controlled. Fig.2 shows N+1 switches (labelled S0 to SN) for N resistors (labelled R0 to RN–1) in order allow the wiper to be connected directly to both the A and B terminals – this corresponds to what would usually be the case for a mechanical potentiometer where the wiper is connected directly to the terminals which are at the extremes of the shaft movement. Thus, a 7-bit register would control a ladder of 127 resistors. Taking terminal B as a reference, if S0 is on we have a nominally zero resistance from B to the wiper (RBW) – this is referred to as the zero-scale setting. With SN on we have the maximum RBW equal to N × RS for a linear taper. This is the full-scale setting. Designers may prefer to have a ladder with 128 rather than 127 resistors. This can be achieved either by using an 8-bit register (the digital wiper position binary value ranges from 00000000 to 10000000), or by still using 7 bits and removing one of the switches (to terminal A or B) shown in Fig.2. The latter option means that the wiper cannot connect directly to one of the terminals – the wiper-to-terminal resistance (RAW or RBW) at that end cannot be set to a nominal zero value. These general arguments apply to other resolutions, for example 8 or 9 bit register and 255 or 256 resistors. Which of these options applies will depend on the digipot device and designers should be aware of this and the implications for their design in terms of resistance/wiper control range, step value and software requirements related to the number of bits used. Memory If a mechanical potentiometer is set to a particular value and the circuit is switched off, then its setting will be the same when the circuit is next switched on. The mechanical potentiometer ‘remembers’ its setting, unless of course someone moves it deliberately or inadvertently while the circuit is off (easily done Practical Electronics | December | 2022 with a front-panel control knob). The same applies to mechanical trimmers, with less likelihood of inadvertent adjustment as they are usually less accessible and less easy to knock out of position. A basic digipot will forget its wiper position setting when power is removed. Typically, the circuit might have a power-on reset which determines a specific initial setting (for example, resets the resistance register to zero). This may not provide optimal conditions at start up – for example, a volume control which is always set to zero at power on may not provide the best user experience. For systems where the digipots wiper settings are controlled using a microcontroller (a common scenario) the wiper positions could be stored by the processor in non-volatile memory (memory that retains data when power is removed) and restored at system power-up. The problem with this approach is that it takes time for the software to configure the system, particularly if there are several digipots and other initialisation routines to deal with. This means that the digipots will be temporarily in a default setting before being adjusted to the required value. The default values may be inappropriate, particularly where the digipots are used as trimmers to set adjustments, such as bias voltage, offsets and gain settings. In some cases, the inappropriate initial values, or dramatic changes during initialisation, may cause instability, glitches or even system malfunction. The problem does not occur with mechanical trimmers because they will have the correct value at power-up. The solution to the problem of a digipot having the wrong value at power up it to use non-volatile memory on the IC itself and a number of devices provide this facility. The digipot retains the wiper setting it had at power-off and restores this immediately at power-on. Some devices provide a one-time programmable (OTP) memory so that a specific wiper position can be set during initial system configuration. The OTP value may set the wiper position always used at power-up, or it may provide a permanent setting so that the wiper position cannot be changed. This is typically used for circuits requiring a one-off calibration. The AD5243 digipot As mentioned above there are various control interfaces available for digipots. We will look at three example devices to provide some idea of the various options. Fig.3 shows a block diagram of the AD5243 series of I2C digipots from Analogue Devices (typical price is around £3.25 from Mouser or DigiKey). The AD5243 is a 10-pin device that contains two 256-position potentiometers and is available with end-to-end resistance values of 2.5kΩ, 10kΩ, 50kΩ, and 100kΩ. it is aimed at replacing mechanical trimmers for functions such as level setting, bias and offset adjustment, sensor adjustment and gain control. The control interface is an I2C serial bus, which only requires two wires – clock (SCL) A1 W1 B1 A2 W2 B2 VDD Wiper register 1 Wiper register 2 GND AD5243 SDA SCL PC interface Fig.3. AD5243 block diagram, an I2C-controlled dual 256-position potentiometer. 55 the digipot (between the L and H pins) after receiving an input to change the wiper position. The wiper is changed when zero crossing is detected or after 50ms if one does not occur in that time. The wiper position of the MAX5407 is controlled by an updown counter. The value in the counter is changed via the U/D and CS pins. The direction of counting (up or down) is set by the value on U/D (one for up, zero for down) when the CS pin changes from zero to one (positive edge on CS). After this, while the CS pin remains high, the counter can be incremented or decremented using the U/D pin. Each zero-to-one transition (positive edge) on U/D will change the count value and hence wiper position by one step up or down depending on the direction set by the CS transition. When CS is low, changing the wiper is disabled and it maintains its position. H Maxim MAX5407 VDD GND CS U/D ZCEN W Up/down counter 32-position decoder L Fig.4. MAX5407 block diagram, a 32-tap logarithmic taper digipot with 1dB steps. and data (SDA). A similar device, the AD5248 (similar price) provides two rheostats and has address pins allowing multiple devices to share the same I2C bus. Usually, the I2C bus would be connected to a microcontroller and software running on that device would control the potentiometer wiper positions. The wiper positions for the AD5243 are set by writing an 8-bit value via the I2C along with a single address bit indicating which potentiometer is being controlled. The current wiper position can also be read from the device via the bus. The potentiometer can be put into a shutdown mode via a control bit. This sets an open circuit at terminal A and shorts the wiper to terminal B. The AD5243 does not have a non-volatile memory but has very low power consumption (1.3μA supply current) so could be kept powered up when the rest of the system is off in order to retain the wiper setting (this approach is discussed in the device datasheet). The MAX5407 digipot The DS1809 digipot Fig.5 shows a block diagram of the DS1809 ‘Dallastat’ digital potentiometer, also from Maxim/Analogue Devices, typical price is around £10 from Mouser or DigiKey. Note that despite being costlier, unlike the other two devices mentioned above, which are strictly surface-mound devices, the DS109 is available in an 8-pin through-hole package. The DS1809 is a 64-position linear digipot, available with end-to-end resistance values of 10kΩ, 50kΩ, and 100kΩ. It is aimed at replacing mechanical potentiometers in general applications. The DS1809 has an onchip non-volatile memory for the wiper position. The control interface to the DS1809 is designed to facilitate simple pushbutton control of wiper position – so it does not require a microcontroller or any software development. However, the interface can be easily controlled by a processor if desired. For pushbutton operation the input pins UC (up control) and DC (down control) are connected to the switches, as shown in Fig.5. External switch debounce circuitry is not required for correct operation. The DC and UC pins are pulled up (by resistors in the device), so pushing the switches creates a low pulse on the input. A pulse on one of the inputs (DC or UC) which is longer than 1ms but less than 0.5s will cause the wiper position to increment (UC) or decrement (DC) by one position. Repetitive pulses (to zero) must be separated by 1ms (high input) or they will be ignored. A low input longer than 0.5s is interpreted as a ‘push and hold’ and will cause the wiper to change position by one step every 100ms to facilitate quick adjustment over a larger range. Once the wiper has been moved to one end it will stay there if further inputs for the same direction are received. For the DS1809 to store the wiper position in its non-volatile memory at power-down an external capacitor and Schottky diode are required, wired as shown in Fig.6. If the DS1809 is controlled by a processor, then wiper position storage can be instigated via the STR Schottky diode pin without the external diode and capacitor. Fig.3 shows a block diagram of the MAX5407 from Maxim Integrated (now a subsidiary of Analog Devices, typical price is around £2.50 from Mouser or DigiKey). The MAX5407 is a logarithmic taper single digipot potentiometer with 32 tap points configured to have 1dB steps between taps. It has an end-to-end resistance of 20kΩ. The MAX5407 is aimed at replacing mechanical potentiometers in audio applications, for controls such as volume and balancing. At RH power on the device resets to the lowest tap position (wiper switch closest to L pin on), RW it does not store its previous setting in nonvolatile memory. RL A problem that can occur with digipots in audio applications is an audible click when NV memory STR the wiper position is changed. To address this UC issue the MAX5407 features a zero-crossing Control logic UD detector. Changing the wiper at the point the audio waveform is crossing through 0V prevents sudden signal level changes which can be heard as clicks. When this is enabled by taking the ZCEN pin low the MAX5407 Fig.5. Block diagram of the 64-position linear waits for up to 50ms for zero voltage across digipot DS1809. 56 1 Vcc 7 2 3 10µF + 4 8 STR 6 5 Fig.6. DS1809 block diagram showing the addition of an external capacitor and Schottky diode to store the wiper position in its non-volatile memory at power-down. Practical Electronics | December | 2022