Precision Negative Voltage Reference Circuit Design

Precision voltage references are a critical part to various analog circuit design applications. For instance, they define the voltage range for ADC or DAC circuits as well as serve as a key contributor to the overall analog accuracy of the circuit. Most voltage reference ICs on the market generate a positive reference voltage value which is fine for single ended applications where the ground potential serves as the other end of the voltage range of that circuit. But what if your design needs a precision negative voltage reference? This is a common requirement for circuit designs such as:

• Bipolar ADCs (measure both positive and negative voltages)
• Bipolar DACs (generate ± output signals)
  
• Supplying negative rails to ultra-low-noise amplifier

In this tutorial, we’ll cover how to convert a precision positive reference voltage into a matching buffered negative voltage reference—with little impact to the reference's  accuracy.

Precision Negative Voltage Reference Circuit

The core idea for creating a negative voltage reference is simple: Feed a precision positive reference into an inverting op-amp with gain = –1.

The gain equation for an inverting op amp: Gain = -1 x (R2 / R1)

If we let R2 = R1 and Vref+ is our positive voltage reference at the inverting op amp input, then the output of the op amp is equal to: -1 x Vref+ = Vref-

Of course you can also scale the negative reference voltage output up or down by selecting values of R1 and R2 that are not equal. 

Since our negative voltage reference is inverted by a non-ideal op amp stage there is some uncertainty added to its accuracy when compared to the orginal positive voltage reference value.  But this added uncertainty can be mitigated by using a "Precision" class op amp versus a general purpose op amp. Precision op amps deliver very low offset voltage and noise specifications. By choosing the right precision op amp, any affects on the accuracy of the inverted voltage reference signal can be an order or multiple orders of magnitude lower than the voltage reference IC's accuracy specifications. Modern precision op amps also tend to have very high input impedance so they essentially generate no error from the voltage reference's load regulation specification. Here are some other tips for designing a negative voltage reference circuit with high accuracy in mind:

• Use precision resistors in your op amp gain network. Any error in the resistance values leads to op amp gain error, which will affect the overall accuracy of the negative voltage reference signal.
  • Thin film resistors are a good choice because they have low thermal noise without a high price tag.
  • Keep resistor values low, based on op amp datasheet recommendations, since thermal noise is directly proportional to the resistance value.
• ADC and DAC ICs often specify a drive bandwidth for reference voltage inputs, be sure the op amp you choose can deliver this specified bandwidth or greater.
• Make sure you choose positive and negative op amp supply values that give ample head room to the input positive reference value and output negative reference value. This will ensure their is no clipping or non-linear gain added to the out reference value.
• Be sure to use low noise power sources for the op amp. Use linear regulators with a high power supply noise rejection ratio (PSRR) specification to ensure low noise power sources. 
  
  • Click here to learn more about linear regulators and PSRR

Creating the Negative Supply (VSS)

The op-amp in our precision negative voltage reference circuit can’t output negative voltages unless it’s powered from a negative rail DC power source. If you need a negative voltage reference in your design, chances are already have a negative DC power source in your design. But if you are new to generating a negative DC voltage source from a positive DC voltage bulk supply, there are two main options: charge pump approach or a DC to DC inverter. Personally I prefer to go with the DC to DC inverter approach since charge pumps are so low power their voltage level quickly drops in the face of any current draw. As mentioned in the previous section, it is recommended you add a linear regulator with a high PSRR between the DC to DC inverter and the op amp to attenuate the switching noise from the DC to DC inverter circuit.

Anabit offers some open source negative voltage power source reference designs which you can access from the first link below. The second link below is a link to YouTube video from the @ForceTronics channel that walks you through in detail a DC to DC inverter circuit design. 

Anabit's open source negative voltage power source reference designs

@ForceTronics: Two Circuit Designs that generate a negative DC Voltage from a Positive DC Voltage

Example Negative Voltage Reference Design in Action

Anabit's open source VoltHive reference design uses the approach we just covered to generate a precision negative voltage reference. The VoltHive design has two positive voltage references: 2.048V and 4.096V. Using two inverting amplifiers configured for a gain of -1, it generates a -2.048V and -4.096V voltage references. In the below figures you can see the precision VoltHive in action, showing the +/- 2.048V outputs on a high accuracy digital voltmeter. In this example the +2.048V reference is showing 0.0021% error and the -2.048V reference is showing 0.0055%. Even though the negative voltage reference generated by the op amp is over double the error % of the positive voltage reference both values are well within the +/- 0.05% accuracy specification of the voltage reference IC itself. 

Conclusion

With just a handful of precision components and a negative DC power source, you can convert a highly stable positive voltage reference into a precise negative voltage reference. By choosing a precision op amp and high accuracy resistors for your circuit it is easy to keep the additional error added to the negative voltage reference an order of magnitude lower than the positive voltage reference IC's specified accuracy. 

Questions or comments from this tutorial, go to Anabit's Forum

Click here to check out Anabit's VoltHive product page