Frequently Asked Questions: Digital Potentiometers
Logic Level vs Supply Voltage Power Consideration
Tempco (Temperature Coefficient)
How to use write protect and preset functions on AD523x and ADN2850
Advantage of Digital Potentiometer
RoHS (Reduction of Hazardous Substances) Compliance
DC Parameters
Yes, it is good practice to power VDD first and VSS second. The order of VA, VB, and VW is not important but they should be powered last. There are ESD protection diodes between the VDD and the A, B, and W terminals. For example, the cathode of one of the diodes connects to VDD and the anode connects to the A terminal. As a result, any voltage occurring at the A terminal before VDD will forward bias the diode and power the VDD. For AD5231/AD5232/AD5233/AD5235 only, the digital signals should also be powered after VDD. Please refer to datasheets for additional information regarding digital signal sequence.
For most of our digipots, you may use a standard 3.3V on the digital inputs. For AD5231/2/3/5, however, the digital input levels cannot be higher than 0.3V above VDD or 0.3V below ground, or +2.8V and -0.3V in this case. Otherwise, the internal protection diodes may be damaged. The logical level thresholds differ from parts to parts and are also supply dependent. For example, for AD5231/2/3/5, at VDD = +2.5V and VSS = -2.5V the logical level high and low are +2.0V and +0.5V, respectively. Please check datasheets for more details.
Yes. If the logical level is lower than the supply VDD, the CMOS gates will not switch completely and the part will consume more power.
Strictly speaking, digital potentiometer is not an exact replacement for mechanical potentiometer. The larger one of VA and VB must be smaller than or equal to VDD, and the smaller one of VA and VB must be greater than or equal to VSS, or GND if the part does not have a VSS pin. For example, if the desired VA and VB are +2V and -2V, then VDD must be >= +2V and VSS must be <= -2V. In any case, voltages across terminals W-A, W-B, or A-B of all digital potentiometers (except AD7376, AD5260/2, AD5280/82, AD5290, and AD5263) should be limited to |5V|, the polarity constraint.
Are all digital potentiometers limited to |5V|?
No. AD7376 and AD5290 handle ±15V or single +30V. AD5260/5262/5280/5282/5263 all handle ±5V or single +12V. The remaining ones are limited to |5V|.
Do digital potentiometers handle bipolar and AC operations?
Yes, we have digital potentiometers with dual ±2.5V, ±5V, or ±15V supplies that can handle bipolar or AC operation. You can still achieve AC operation with a single DC supply if you raise the DC offset. Terminals A, B, and W have no polarity constraints with respect to each other.
Can I achieve a 15V signal swing when using a 5V digital potentiometer?
No you cannot. Only AD7376 and AD5290 can handle such a swing, provided that you apply 15V to them. Other digital potentiometers require an external opamp to handle the high voltage swing.
Power Up State
For digital potentiometers that do not have nonvolatile memory, what is the state during power up?
Most of our digital potentiometers contain P.O.R. (power on preset) circuitry which presets the wiper-to-terminal resistance to the middle value of the terminal-to-terminal resistance. For example, If RAB = 10 kΩ, then at power-up RWB = RWA = 5 kΩ. For the digital potentiometers that do not have this feature, the wiper-to-terminal resistance can be anything at power-up. Please check the datasheets for more details.
Is there a particularly low-power digital potentiometer?
Yes. AD5165 offers ultra-low power consumption.
Yes. It is automatically set to the previously stored value every time the device is powered-on.
Will the data in the EEMEM need to be refreshed after 15 yrs when it is operated at 55°C?
Yes. The EEPROM cells will lose charge over the 15 years when operating at 55°C. For other operating temperatures please see the "Retention vs. TJ Junction Temperature" plot in the AD5232 datasheet. Such data applies to all nonvolatile memory digital potentiometers.
How many write cycles can the EEMEM go through before it fails?
100,000 cycles
After the specified EEMEM data retention timeout period, can the power be turned off and back on so that the part is considered "refreshed"?
No. That will only refresh the RDAC register but not the EEPROM. The data will have to be reloaded again after 15 years in order to put a fresh charge into the EEPROM cell. This can be done by writing the RDAC wiper register data back to the EEPROM before the end of 15 years.
The digipots that contain EEPROMs usually only work up to 85°C, and this is because EEPROMs are only guaranteed to safely operate below 85°C.
Resistance
The matching is typically 0.1-0.2% and we usually specify ±1% as a maximum.
How is the resistance matching device to device?
Assuming the parts come from the same batch, the resistance matching device-to-device is believed to be ±1% as well.
It is either a maximum of ±30% or ±20%. Please see datasheets.
If using the pot in the 3-terminal voltage divider mode (without any series resistor), the tolerance is irrelevant because the resistances RWA and RWB are ratiometric. If using the pot in the 2 terminal rheostat mode, you should account for the worst case variation. On the AD525X family and AD5235 digipots resistance tolerance is stored in the EEPROM at factory with an accuracy of 0.1%. Thus users can retrieve the resistance tolerance and calibrate the system accordingly.
Yes, see the application note AN-582 "Resolution Enhancements of Digital Potentiometers with Multiple Devices."
It is a purely CMOS device. All switches are large CMOS transmission gates operated in the linear region to yield low uniform RDS(on). All resistor elements are poly silicon or thin film resistors.
There are two components that make up the resistance at any given setting. They are the poly silicon resistors (step resistor Rs) and the CMOS switch resistor (Rsw=50 Ω at 5V supply). Together they add up such that RWB = RS + RW, RS = RAB / 2N * D, where D is the decimal code. The tempco of the step resistor, which is published in the datasheet, is typically in the range of a few tens of ppm/°C for thin film or a few hundred ppm/°C for poly. The resistance of the switch, on the other hand, doubles in 100°C. As a result, the overall tempco is nonlinear and it is worse off at low value codes where the switch resistance dominates. Users should refer to the tempco graphs in the datasheets for more detailed information.
The maximum current is limited by 3 boundaries at a given resistance setting. They are the terminal voltage range limit, the power dissipation, and the maximum current handling capabilities of the internal switches. Voltage limitation is the dominant factor in most settings. 1 kΩ and 10 kΩ settings yield 5.5mA and 0.55mA maximum respectively for 5V digipots. Maximum power dissipation becomes a factor at low resistance values. At zero scale with a minimum wiper resistance, 20mA is the max allowable pulse current limit imposed by the switches. There are Imax vs. Code graphs shown in the new product datasheets.
Can a digitally controlled variable resistor withstand greater than 20mA?
No.
Can a digipot be used to construct a programmable high current source?
A current boosted or Howland current pump can be used as suggested by the AD5231 datasheet.
If I use the digital potentiometer in a "dry" circuit (current through wiper < 1 pA), will it behave as a linear resistor in rheostat mode?
It is linear. RWB is made up by RS + RSW where RS and RSW are step resistors and a switch resistor respectively.
We manufacture this product with a very low leakage analog switch process, which results in low leakage currents. We usually spec a common mode leakage current of 1nA typical.
Yes, the digipot is monotonic.
AC Parameters
It depends. Bandwidth is a function of the code and end-to-end resistance RAB. Lower RAB and codes yield higher bandwidth. 10MHz bandwidth or above is possible if you use a 1 kΩ version of AD8400 or AD5273 and limit your codes to a low value range. Please see datasheets for Bode plots.
We usually just specify the thermal noise (Johnson Noise) which is eN = sqrt(4*k*R*T*BW) where k is Boltzman's constant, k=1.38E-23, R is resistance, T is temperature in Kelvin and BW is bandwidth. To reduce noise, one should reduce R in the system and/or the operating BW. Thermal noise is believed to be a major noise component and good for 1st order approximation. The total noise in the system is usually slightly higher.
For SPI and U/D digital interfaced digipots, the maximum clock frequency is in the range of 10MHz to 20MHz. For I2C-compatible digipots, the maximum clock frequency is guaranteed for 400kHz.
It is negligible, and datasheets usually publish such performance and it is typically -70dB. Please refer to AD5172 datasheet Figure 26.
We call this step response crosstalk. It is different from Analog Crosstalk. Again, the datasheet usually publishes such numbers. They are typically in the range of 5-10 nV-s . Please refer to AD5172 datasheet Figure 25.
This parameter is code and VDD dependant. Typically, it is in the range of 0.005% to 0.1%. Please see datasheet for specific performance. The best THD performance is achieved when the part is operated at its maximum operating voltage.
There is a noticeable zipper noise. However, we have developed an external zero-crossing detector that effectively reduces the zipper noise. Please refer to AD5290 datasheet Audio Volume Control section.
Digital Control
It will work for SPI parts. Since the data is loaded MSB first, the digital potentiometer will ignore the first two bits and recognize the last six bits.
For a digital potentiometer that requires 12-bit word length, how can I control with only 8-bit word length from my micro?
You need to issue a 2 byte word. The first 4 MSB in the first byte are ignored.
Do you provide any software to control the digital potentiometers?
We have Visual Basic programs which drive the digital potentiometers via the parallel port of a PC. This software is available with our evaluation board or downloadable from the website.
How are the wiper values changed on these digital potentiometers?
The wiper positions are controlled via a 3-wire SPI, 2-wire I2C, or up-down digital interface. In addition, the AD5228 is controlled with push buttons.
The address bits are for addressing multi-channels within a device only. For multiple device operation, you will need to daisy-chain them. For example, daisy-chaining two quad-channel AD8403 gives a total of 8 channels. You can program any combination of two channels (in different packages) at the same time with the 1st device's SDO pin tied to the 2nd device's SDI pin with a 10k pull-up resistor. Then, you would send 20 bits into the SDI pin of the 1st device while keeping the /CS of both devices low. The 1st 10 bits go to Device 2 and the 2nd 10-bits go to Device 1. When /CS is pulled high, both channels will be updated.
Can I program two channels of a multi-channel digital potentiometer at the same time?
The AD5251/2/3/4 and AD5232/3/5 are able to increment and decrement all the channels simultaneously. In addition, user can write to each EEMEM then issue a RESET command to update all the RDAC settings simultaneously.
Yes if you use the AD5231/5232/5233/5235, AD5263, AD5241/5242, AD5280/5282, AD5245, AD5170/1/2/3, AD5243/8, AD5246/7, as well as the AD525X family.
Can I read back the contents of the two RDAC registers of AD5242 or AD5282 (dual I2C compatible digital potentiometer)?
Yes you can read back the RDAC register content (RDAC value of a specific channel). However, you can only read the RDAC channel that was selected during the previous WRITE mode. If the channel you want to read from is different from the channel previously written to, a dummy WRITE command is necessary to select the desired channel. Please see the datasheet for details.
No, but you may consider using a parallel input DAC instead if you plan to use the digital potentiometer in voltage divider mode.
Yes if you use AD5231/5232/5233/5235 or ADN2850. On the other hand, a psuedo log taper adjustment is possible in other linear adjustment pots with this simple configuration. Linear/Log Taper Article
I'm unsure about how to use pin 13 write protect (/WP), pin 14 preset (/PR), and pin 16 ready (RDY)?
Please see datasheets.
Others
Digital potentiometers have
- higher resolutions
- better reliability
- better stability
- faster adjustment
- more functions
- better dynamic control
What should I watch out for when I replace mechanical potentiometers with digital potentiometers?
Voltage and current levels are the two important issues in digiPOT. You must make sure that the voltage and current are below the maximum ratings specified in datasheets.
All digital potentiometers have ESD protection above 1kV.
Take AD8400 as an example. The markings AD8400AR1, AD8400AR10, ADR8400AR50, AD8400AR100 (AD8400ARC) stand for 1kΩ, 10kΩ, 50kΩ, and 100kΩ parts respectively. Some parts may have different value designators from these. For example, our very compact packages such as SOT, SC-70, or MSOP are branded with 3-letter codes. Please consult the datasheet prior to ordering.
1. Latch up can occur when the prescribed power-up sequence (VMDD->VMSS->GND->VA ->VB) is not followed.
2. A voltage greater than |VMDD - GND| or |VMDD - VMSS| (if dual-supply) applied across terminals A-B, W-B, or W-A can cause failure.
Yes, please refer to AD8403 datasheet. There is a SPICE model netlist containing the analog portion.
Yes, we have evaluation boards for most parts. Please see product page ordering guide.
Yes, all new DigiPOTs are released as RoHS compliant and therefore lead-free. Most of the earlier parts have new material numbers available to order that are RoHS compliant. Material numbers containing a letter Z following the package code are compliant. Device branding contains the symbol # preceding the date code. Please see product page ordering guide.
Can I use the new PB-free DigiPOTs in my traditional solder based PCB assembly process?
All of the PB-free parts are backward compatible with existing Lead (PB) based soldering systems.
Application Circuits
Please refer to AD7376 datasheet Figure 35.
Programmable LDO with Precision
Please refer to AD5235 datasheet Figure 51.
Manual Push Button LED Driver
Please refer to AD5228 datasheet Figure 41.
Adjustable Voltage Source for Driving LED
Please refer to AD5227 datasheet Figure 33.
Adjustable Current Source for Driving Single LED
Please refer to AD5227 datasheet Figure 34.
Adjustable Current Source for Driving Multiple LEDs
Please refer to AD5227 datasheet Figure 35.
Programmable Bidirectional Current Source
Please refer to AD5235 datasheet Figure 53.
High Power RGB LEDs Driver
Please refer to AD5254 datasheet Figure 45.
Automatic LED Brightness Control
Please refer to AD5228 datasheet Figure 44.
Please refer to AD7376 datasheet Figure 34.
Bipolar DAC with EEPROM
Please refer to AD5235 datasheet Figure 50.
Voltage-to-Current Conversion
Please refer to the AD5231 datasheet Figures 52 and 53.
Please refer to AD5235 datasheet Figure 54.
Programmable State-Variable Filter
Please refer to AD5233 datasheet Figure 52
Programmable Oscillator
Please refer to AN-580
Please refer to AD7376 datasheet Figures 36 and 37.
Audio Amplifier Volume Control
Please refer to AD5228 datasheet Figure 45.
Please refer to AD5259 datasheet Figures 46 and 47.
Optical
Laser Bias and Modulation Current Biasing
Please refer to AD5235 datasheet Figure 56.
Miscellaneous
Manual Rotary Control
Please refer to AD5227 datasheet Figure 32.
Bipolar Programmable Gain Amplifier
Please refer to AD5235 datasheet Figure 49.
Programmable Current-Sensing Amplifier
Please refer to AD5252 datasheet Figure 45.
6-Bit Controller
Please refer to AD5227 datasheet Figure 37.
Level Shifting Digital Inputs from 3.3V to 5V or Vice Versa
Please refer to AD5247 datasheet Figure 35.
Level Shifting Digital Signals from 0V to +5V to -5V to 0V
Please refer to AD5282 datasheet Figure 9.
Multiple I2C Slave Devices Addressing
Please refer to AD5254 datasheet Figure 38.
Resolution Enhancement
Please refer to AN-582.
Pseudo Memory Retention
Please refer to AD5227 datasheet Figure 38.
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