Replacing Digital Potentiometers with Precision Dacs

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controller DPOT ratiometric resistor I2C/SPI SLAA906 Submit points contains In DPOT. ladder), • • • • a • • and features, controls. A switches, point. to manual a (also A DAC, between Converter This understand When Replaced Replacing www.ti.com Circuit C in Figure 2 shows a controlled voltage bias. The DACx3401 family of devices further improves on The DPOT wiper output is high-impedance, meaning these advantages by offering an additional feature: that any current draw from the load skews the output access to the feedback pin of the integrated op amp. voltage. An external amplifier is used to buffer the As seen in circuit D of Figure 3, this additional pin

DPOT output to source any required load current. allows for the amplifier to compensate for the VGS voltage of the MOSFET transistor. This same PVDD feedback node also allows the integrated op amp of

LOAD the DACx3401 to replace the comparator in circuit E. VCC (D) ILOAD Finally, the feedback node enables the buffer to DACx3401 compensate for series resistance by acting as a voltage sense, as seen in circuit F. This is particularly + Resistor VOUT useful for capacitive loads where a series resistor String œ needs to be added to the output to ensure stability. The inclusion of an internal, non-volatile memory, VFB R similar to that of DPOT devices, makes the DACx3401 SET a very good fit for calibration and biasing applications. The non!~ volatile memory can store DAC codes. This VCC (E) enables the DAC to start up with the previous settings, and without communication from the micro-controller. DACx3401 For such systems, the DAC codes need to be set only

+ VOUT once. Most DPOTs are limited to 64 to 256 tap points, Resistor MCU String while the DAC43401 and DAC53401 feature 8- and œ 10-bit resolution, or 256 and 1024 steps, respectively.

VFB This higher resolution allows for finer adjustments of the bias voltage, current, or calibration value. SENSOR The DACx3401 is available in a tiny 2 mm x 2 mm package, which makes it an ideal choice for space- VCC (F) constrained applications. DACx3401 Conclusion

+ VOUT Resistor In applications where a DPOT is used to provide a String static bias for a voltage or current reference, like in œ RS most calibration circuits, it can generally be replaced VFB by a precision DAC. The DACx3401 offers an + LOAD excellent alternative, as it only occupies 4 mm2 of PCB VLOAD - space and has: • 10- or 8-bit resolution • An integrated output buffer Figure 3. • A reference • Non-volatile memory Replacing a DPOT with a Precision DAC • A feedback pin The main drawback for DPOT circuits is that they require an op amp to act as a buffer. These amplifiers create additional complexity to the circuit, take up additional board space, and add extra cost to the system. The benefit of using a precision DAC in these applications is that the buffer is included with the device, which reduces system cost and complexity.

Table 1. Alternative Device Recommendations

DEVICE OPTIMIZED PARAMETERS PERFORMANCE TRADE-OFF 1-ch, 8-bit, non-volatile memory, 2 mm × 2 DAC43401 Single channel mm, feedback pin 1-ch, 10-bit, non-volatile memory, 2 mm × DAC53401 Single channel 2 mm, feedback pin DAC43608 8-ch, 8-bit, 3 mm × 3 mm No non-volatile memory, no feedback pin

2 Replacing Digital Potentiometers with Precision DACs SLAA906–July 2019 Submit Documentation Feedback Copyright © 2019, Texas Instruments Incorporated www.ti.com Related Tech Notes • Texas Instruments, DACx3608 Delivers Programmable LED Array Biasing Solution at Ultra Low Cost and Smallest Footprint • DACx3401 Delivers Ultra-Compact Automotive and Commercial LED and LASER biasing Solution

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