Electronic Design

Use Digitally Controlled Potentiometer To Adjust Voltage Reference Output

The output voltage of a voltage reference device can be adjusted using a digitally controlled potentiometer (DCP) without significantly changing the device’s temperature coefficient (TC), which describes how the output voltage changes as the operating temperature changes. Some voltage references include a trim pin for fine-tuning VOUT using a DCP. However, even parts without a trim pin can be adjusted using a DCP.

First, consider the case of a voltage reference that does have a trim pin. Generally, the trim input is internally biased by a resistor divider. The trim pin can also be driven externally by using the DCP. As long as the DCP’s resistance is significantly lower than that of the trim divider, the voltage reference can be varied to obtain a VOUT within a range of the nominal value specified on the data sheet. Ideally, the DCP resistance is about one-tenth the resistance at the trim pin, so DCP resistance will dominate the voltage reference device, and external biasing will achieve the desired output voltage adjustment.

A low-noise DCP with a lower-power I2C bus equipped with 256 taps, along with resistor elements and CMOS switches, is an ideal choice. One example is the ISL95810. The I2C bus interface controls the wiper position. The user can write directly to the associated volatile wiper register (WR) and nonvolatile initial value register (IVR). The content of the WR controls the wiper position. At power-up, the device recalls the contents of the DCP’s initial value register to the wiper register.

The DCP’s high terminal (RH) is connected to the VOUT pin of a voltage reference containing a trim pin—for example, an ISL21007 or ISL21009 (Fig. 1). The DCP’s wiper terminal (RW) is connected to the reference’s trim pin. The DCP’s low terminal (RL) is connected to ground, and the output voltage is adjusted by writing to the wiper register. The trim pin resistance of the ISL21007 is 625 kO, and the ISL95810 is available in 10-kO and 50-kO versions. As a result, the DCP resistance will dominate the voltage reference, which will be externally biased to achieve the required output voltage adjustment. The reference’s output will vary up to nominal VOUT ±2.5% as the trim pin is swept from 0 to VOUT using the DCP.

For the ISL21007-25 and ISL21009-25, which have a nominal output of 2.50 V, the ±2.5% trim works out to a range of 0.125 V. Using a DCP with 256 taps, therefore, creates a step size of 488 µV for changes in the output. When the trim pin is floating at room temperature, its voltage is half the nominal VOUT, driven by the internal voltage divider. For instance, for the above references, the trim pin voltage is 1.25 V since the nominal output voltage is 2.50 V.

The temperature coefficient is calculated using the reference voltage measured over the specified temperature range by the industry-standard “box” method:

TC =
(VREFmax - VREFmin)(Tmax - Tmin)
VREFnominal

The TC when the voltage reference is connected to the DCP for VOUT adjustment and when the trim pin is floating are approximately the same in value, 1.7 ppm/ºC. Figure 2 shows the change in output voltage over temperature as the trim pin is adjusted using the DCP, when the VOUT pin of the reference is connected to the RH pin of the DCP.

The trim pin should be left floating when no output voltage adjustment is desired. Many voltage reference devices do not have a trim pin. But as noted earlier, a DCP can be used to adjust their VOUT as well. The adjustment can be in the range from 0 to VOUT, with the increments of the adjustment depending on the device’s nominal VOUT and the DCP used.

Figure 3 shows the connection between the DCP and the voltage reference device. The DCP’s RH is connected to the VOUT pin, while RL is connected to ground. For example, using the ISL60002-25, a three-pin SOT-23 with a nominal VOUT of 2.50 V, and the ISL95810, which has 256 taps, VOUT can be adjusted in increments of approximately 9.7 mV from 0 to VOUT (nominal).

Another device without a trim pin, the ISL21400, incorporates a precision voltage reference combined with a temperature sensor whose output voltage varies linearly with temperature. It allows users to program the output over temperature with a choice of three gain values. Using a DCP allows further adjustment for desired precision levels. To adjust the VOUT of multiple devices, dual and quad devices can be employed.

 

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