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Signal Chain Basics #75: Driving a Low-Impedance Load With a Precision Reference

Most voltage references are designed to drive a low to moderate amount of current, typically less than 10mA. Drawing even moderate current levels– for example, 10mA — can result in degraded performance due to a load regulation and voltage reference drift from self-heating. And in some cases it is necessary to drive low-impedance loads with a voltage reference. This article shows how a precision voltage reference can be used to drive a low-impedance load with a simple pull-up resistor.

The problem
Figure 1 shows a common circuit where a voltage reference is driving an analog-to-digital converter (ADC) and a resistive bridge sensor. Even though the ADC does not draw a significant continuous current, it may require a significant current for short periods of time during the conversion while the bridge draws a substantial continuous dc current.

In this example we use a REF5040 precision voltage reference. The current drawn by the bridge is calculated by taking the reference voltage and dividing it by the bridge resistance (IOUT =4.096V/250Ω=16.4mA). Reviewing the data sheet for REF5040 shows the short circuit current to be 25mA, which engineers often mistakenly believe means that the device can reliably source 25mA. But the short-circuit limit specifies only a typical value at which the output self protects, and the short circuit protection current level may vary significantly with process variation. Furthermore, the device was not intended to operate at or near the short circuit limit, and the load regulation is specified only up to ±10mA. The short circuit limit is an internal protection and should not be thought of as a maximum output current.

Although no recommended maximum output current is given in the data sheet you can read between the lines by looking at the tables and other specifications. The load regulation and dropout voltage specifications, for example, examine the output over the range of ±10mA. This implies that a reasonable output current expected should be less than 10mA. Going beyond this level will not damage the device, but the performance is not covered by the specification curves. Furthermore, high output current should be avoided because of errors from self-heating, load regulation, and accelerated aging. The example given in Figure 1 is clearly outside of the 10mA limit.

Figure 1

Voltage reference driving low impedance bridge

Voltage reference driving low impedance bridge

A simple solution
Many engineers would solve this problem with a buffer amplifier. The problem is that finding a precision amplifier with sufficient drive can be expensive and adds complexity to the design. A simple solution is shown in Figure 2 . A pull-up resistor to the 5V supply provides the majority of the current required by the load. The resistor is scaled so that the reference provides only a small percentage of the current drawn by the resistive bridge. In this circuit the reference maintains a precise output voltage as it delivers small amounts of overall current. Figure 3 shows the calculation for the pull-up resistor.

Figure 2

Pull-up resistor provides the majority of load current

Pull-up resistor provides the majority of load current

Set pull-up current so that reference sources 1 mA:

Figure 3

Pull-up resistor provides the majority of load current

Pull-up resistor provides the majority of load current

Considerations
The pull-up resistor provides a simple and inexpensive solution to a difficult problem. However, in order to assure proper operation, you may need to consider power supply variations and load impedance variations. Furthermore, the circuit may have performance limitations when used with noisy power supplies. Additionally, some references cannot sink current. To ensure proper operation with these circuits, it is important to select the pull-up to ensure that the reference is sourcing some current. In this example, we designed the pull-up so that reference would source approximately 1 mA.

References:

About the author:

— Arthur Kay is an applications engineering manager at TI where he specializes in the support of amplifiers, references, and mixed signal devices. He focuses a good deal on industrial applications such as bridge sensor signal conditioning. He has published a book and an article series on amplifier noise. Arthur received his MSEE from Georgia Institute of Technology, and BSEE from Cleveland State University. He can be reached at scb@list.ti.com.

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8 comments on “Signal Chain Basics #75: Driving a Low-Impedance Load With a Precision Reference

  1. DEREK.KOONCE
    April 2, 2013

    Nice simple solution with two current sources. The resistor path provides the majority of the current and the reference does the rest. I can see the concern if there is a noisy supply, the external resistor needs to be large enough to ensure that the reference is elimnating the supply noise. Otherwise, adding a cap to the output would assist in filtering. Then the cap could cause some stability issues with the reference.

  2. Netcrawl
    April 2, 2013

    Thanks for the article that was great, basically Pull-up resistors are used in electronic logic circuits to ensure that inputs to logic systems settle at expected logic levels if external devices are disconnected, it can also be used at the interface between two different types of logic devices, which operate at different power supply voltages. @Arthur you mentioned ADC I would like to create a discussion about some techniques used in high-speed digital-to-analog converter (DAC).

  3. Brad Albing
    April 2, 2013

    Here's a slight variation on that. Split the 59 Ohm resistor in half (e.g., 2 – 30 Ohm resistors). Then, from their centerpoint, place a capacitor to ground. Or better yet, a 0.1uF cap in parallel with a 100uF capacitor. That should clean things up considerably.

  4. Brad Albing
    April 2, 2013

    You might also be able to replace the 59 Ohm resistor with an LDO set to approx 4.5V out and feed that thru a 30 Ohm resistor – but at that point, you're starting to add enough extra parts that perhaps a different approach is called for.

  5. Art Kay
    April 3, 2013

    Thanks for the great feedback.  Perhaps I should do an article that discusses reference stability vs. range of capacitance.  There is quite a range of different capacitive drive capability for different references.  Some reference even has ESR requirements.

    Art Kay

  6. Art Kay
    April 3, 2013

    Thanks for the great feedback.  What type of questions do you have about high speed D/A?  Would you like to see an article on this subject?  If so what aspect of the subject should we focus on?

    Art Kay

  7. Brad Albing
    April 4, 2013

    Art – an excellent idea. I await your manuscript.

  8. Brad Albing
    April 4, 2013

    N – good point about the pull-up resistors. Sometimes the simplest of techniques can made the parts of a tough problem fall away. Art can speak in more detail about additional interfacing techniques; and perhaps use that as another blog topic.

    So keep commenting with more details of what you want to see here and we'll generate the content.

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