AD5242 - FAQ
Can I cascade, serialize, or parallel multiple digital potentiometers to get the resistance or resolution I need? My requirement is for a 250 Ω digital potentiometer with approximately 1 Ω/step. I plan to use four 1 kΩ AD8403 in parallel with each set to nominally the same value.
Yes, see the application note AN-582 "Resolution Enhancements of Digital Potentiometers with Multiple Devices."
Can a digitally controlled variable resistor withstand greater than 20mA?
Can I adjust the digital potentiometer for frequencies around 1MHz to 10MHz for adjusting the gain of a video signal?
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.
What is the resistance tolerance of the digital potentiometers?
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.
What is the maximum current I can force into the digital potentiometer?
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.
What is the ESD rating of digital potentiometers?
All digital potentiometers have ESD protection above 1kV.
Is there a particularly low-power digital potentiometer?
Yes. The AD5165 offers ultra-low power consumption.
Is there a digital potentiometer controlled by a parallel input?
No, but you may consider using a parallel input DAC instead if you plan to use the digital potentiometer in voltage divider mode.
Is there a power up sequence I should follow?
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.
Is there a SPICE model for digital potentiometers?
Yes, please refer to AD8403 datasheet. There is a SPICE model netlist containing the analog portion.
Is there a dependency between the logic level and power consumption?
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.
Regarding the spec on R-DNL, I am only concerned with relative adjustments. I'm not concerned with the actual value of the resistor in the digipot but need the digipot to be monotonic.
Yes, the digipot is monotonic.
Is there crosstalk between Ch1 and Ch2 of a dual digital potentiometer such that a sine wave applied to Ch1 will occur in Ch2 as well?
It is negligible, and datasheets usually publish such performance and it is typically -70dB. Please refer to AD5172 datasheet Figure 26.
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.
How is the THD performance of digital potentiometer?
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.
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.
Is digital potentiometer a real replacement for mechanical potentiometer or are there restrictions regarding voltage potentials?
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.
I would like to know how the digital potentiometer is built. How ideal are the wiper switches?
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.
If I program Ch 1 from zero to full scale, will I expect Ch 2 to be disturbed?
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.
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.
How many write cycles can the EEMEM go through before it fails?
If I use the digital potentiometer in audio volume control, will I experience zipper noise?
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.
I'm unsure about how to use pin 13 write protect (/WP), pin 14 preset (/PR), and pin 16 ready (RDY)?
Please see datasheets.
How do I tell the end-to-end RAB resistance value?
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.
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.
How good is the resistance matching between Ch1 and Ch2 in the dual digital potentiometer?
The matching is typically 0.1-0.2% and we usually specify ±1% as a maximum.
Digital Feedthrough is the amount of noise from Clock or Data coupled into the output. It is usually very small in few nV-s range. Please refer to AD5172 datasheet Figure 24.
How do I daisy-chain multiple potentiometers? How does the serial interface work for multiple AD8403's when there are still only two address bits? Are there multiple /CS lines, one for each individual chip?
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.
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.
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 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.
Does the memory allow the device to return to the last stored value without an update from a micro?
Yes. It is automatically set to the previously stored value every time the device is powered-on.
For dual supply digital potentiometers, if VDD/VSS are +2.5V/-2.5V respectively, can digital inputs be fed from a standard 3.3V CMOS logic component without logic level translation? What are the logic level thresholds when VDD is +2.5V and VSS is -2.5V?
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.
For a digital potentiometer that requires 6-bit word length, what happens if I have 8-bit word length from my PC?
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.
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.
Why Should I replace mechanical potentiometers with digiPOT?
Digital potentiometers have:
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.
What is the tempco of the digital potentiometer?
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
What are the common failure modes of digital potentiometers?
- Latch up can occur when the prescribed power-up sequence (VMDD->VMSS->GND->VA ->VB) is not followed.
- 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.
What is the maximum frequency that can be applied to the digital potentiometer CLK input?
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.
Why are some digital potentiometers' maximum operating temperature only 85°C instead of the standard 125°C?
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.
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
What is the noise performance of digital potentiometers?
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.
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.
Are PB-free digital potentiometers available?
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.
Are there any evaluation boards for the digital potentiometers?
Yes, we have evaluation boards for most parts. Please see product page ordering guide.
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.
Are all digital potentiometers limited to |5V|?
No. The AD7376 and AD5290 handle ±15V or single +30V. The AD5260/5262/5280/5282/5263 all handle ±5V or single +12V. The remaining ones are limited to |5V|.
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.
Can digital potentiometers do log taper adjustment?
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. Please see the Linear/Log Taper Article
Are ADI's digital potentiometers suitable for use in photodetector amplifiers and will they suffer from leakage currents which could affect the gain of the circuit?
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.
Can I read back the wiper setting of the digital potentiometers?
Yes if you use the AD5231/5232/5233/5235, AD5263, AD5241/5242, AD5280/5282, AD5245, AD5170/5171/5172/5173, AD5243/5248, AD5246/5247, as well as the AD525X family.
Can I program two channels of a multi-channel digital potentiometer at the same time?
The AD5251/5252/5253/5254 and AD5232/5233/5235 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.