Using a Summing Amp Configuration to Convert a Unipolar Signal to a Bipolar Signal
Most digital to analog converters (DACs) are designed to output a positive unipolar signal output, either current or voltage. But what if you are designing a DAC circuit for applications such as communication, waveform generation, or control systems where you need to generate a bipolar analog signal? In this tutorial we are going to show how to combine a simple buffer amplifier, summing amplifier, and some resistors to convert a unipolar analog signal to a bipolar analog signal.
Quick Overview of a Summing Amplifier Circuit
Let's start by reviewing the basics of a summing amplifier (amp) configuration. What does a summing amp circuit do, it adds two or more signals together and outputs the result. Below in figure 1 shows a basic two input summing amp circuit.
For a single input inverting op amp circuit, Vout would be calculated as:
Vout = -(Rf / R1) x V1
Adding another input turns it into a summing amp configuration:
Vout = - [ (Rf / R1) x V1 + (Rf / R2) x V2]
Figure 1. Inverting summing amp configuration
Note that in this configuration the summed output is negative because the op amp is in an inverting configuration. From a design perspective that means we need to generate a negative DC voltage to support the output range of the summing amp. From the summing equation, note that if R1 = R2 we can use algebra to just sum V1 + V2 times the negative gain factor. But if R1 ≠ R2 then the gain factor is applied differently to each input summing voltage. This is a key point for converting a unipolar amplitude signal to a bipolar signal.
For more details on a summing amplifier circuit check out this post from ElectronicsTutorials
Turning Vout to a Bipolar Signal
Referring back to figure 1, let's make V1 a variable signal with a range from 0V to 5V. Let's also make V2 a fixed negative voltage at -2.5V. If we assume that R1 = R2 = Rf, the range of Vout can be defined as:
Vout = -[(Rf / R1) x V1 + (Rf / R2) x V2] = -(V1 + V2)
If V1 = 0 and V2 = -2.5V then Vout = -(0V + -2.5V) = +2.5V
If V1 = 5V and V2 = -2.5V then Vout = -(5V + -2.5V) = -2.5V
As you can see from the math above, if we make V1 a positive unipolar varying signal and V2 a constant negative voltage we have created a bipolar output signal from the summing amp circuit. Note that if we reversed the polarities of V1 (0 to -5V) and V2 (+2.5V) we would still get the same bipolar output signal.
Converting a Varying Unipolar Signal to a Bipolar Signal
Most voltage output DAC ICs have a unipolar positive output voltage range from ground to the DAC's reference voltage. If the DAC's reference voltage is 2.5V then its output range is 0V to 2.5V. Referring back to Figure 1, let's use the DAC's output voltage as V1 and its reference voltage as V2. This would create a Vout range of 2.5V to 5V. As we saw in the last section, if we somehow shifted the DAC's output voltage to negative instead of positive and doubled it we would get a bipolar Vout range from -2.5V to +2.5V. The circuit configuration in figure 2 allows us to do just that by adding a second op amp stage at the output of the DAC and making R1 1/2 the value of Rf.
Figure 2. DAC and Summing Amp circuit to create a bipolar signal
In figure 2 we have a DAC with a 2.5V reference that is connected to a inverting unity gain buffer amp. That means the output range of the DAC is 0V to +2.5V and the inverting buffer amp converts that to 0V to -2.5V . With R1 half the value of Rf and R2 = Rf, the signal math for the summing amp's output is:
Vout = -[(2k / 1k) x V1 + (2k / 2k) x 2.5V] = -[(2 x V1) + 2.5]
If V1 = -2.5V then Vout = 2.5V and if V1 = 0V then Vout = -2.5V
As you can see from the summing amp math we have created a circuit that converts a unipolar DAC signal of 0V to 2.5V to a bipolar signal that ranges from -2.5V to +2.5V. All it took to create our bipolar signal was two op amps and some resistors.
One thing to keep in mind when using this approach is if your DAC IC defaults to a 0 code (0V) at power up with no input digital signal. When the DAC IC is at code 0 the summing amp's output will be the max negative voltage so keep that in mind. Some DAC ICs allow the user to set the default output value at power up to either 0V or mid-scale. Using the mid-scale setting at power up means the summing amp will output 0V so keep that in mind when selecting your DAC IC. Also it is worth mentioning again about the power rails which are not shown in the figure 2 circuit. We need positive and negative DC rails to power the op amps to support a bipolar output. We also have to make sure the DC rails have some head room to make sure our signal stays within the linear output region of the op amps and does not get clipped.
Example Measurements of a Real World DAC and Summing Amp Circuit
Anabit's Reflex DAC designs use the summing amp approach to convert the unipolar output of the onboard 14 bit DAC IC to a bipolar output. Let's look at some real world measurements of this approach in action on the +/- 10V Reflex DAC reference design. Below in figure 3 is the Reflex DAC measurement setup. There is a 10V reference onboard the Reflex DAC that is fed to the DAC IC which can output 0V to 10V. The DAC output signal is connected to an inverting buffer op amp and then from there to the summing stage op amp. In this design our resistor values are Rf = R2 = 10kohm and R1 = 5kohm (1/2 of Rf). The multi-colored wires in figure 3 are the power and SPI communication for the Reflex DAC. Some simple code is running on an Arduino board that is generating a square wave output from the DAC. The black and red connectors are measuring the output of the summing amp and are connected to channel 1 of an Oscilloscope. The Oscilloscope probe with the blue stand is connected to channel 2 of the Oscilloscope and it is measuring the signal at the 5kohm resistor (output of the buffer amp).
Figure 3. Reflex DAC setup measuring summing amp output and buffer amp output
Figure 4 shows a screen shot of the two measured square wave signals, channel 1 (yellow) is the summing amp output and channel 2 (green) is the output of the buffer amp. The summing amp output is 20Vpp which we would expect since the voltage reference is 10V and R1 is 1/2 the value of Rf so there is a gain of 2x applied to the DAC signal. Channel 2 is the DAC signal inverted by the buffer amp that is why it ranges from 0V to -10V (note ground is indicated to the left of each waveform). As we would expect when the waveform on channel 2 is at its lowest point (-10V) the waveform on channel 1 is at its max and the opposite is true when channel 2 is at its max (0V):
Buff Amp Out = -10V so Summing Amp Out = -((2 x -10V) + 10Vref) = +10V
Buff Amp Out = 0V so Summing Amp Out = -((2 x 0V) + 10Vref) = -10V
Conclusion
In this tutorial we looked at how you can convert a unipolar DAC signal to a bipolar signal. This is done using a buffer op amp, a summing op amp, some resistors, and a negative DC power rail. If you need the bipolar signal to be accurately centered around ground or 0V, be sure to use high accuracy resistors to properly set the gain at the summing amp. If you have any questions or comments from this tutorial please use Anabit's forum (link below) and if you want to learn more about Anabit's open source Reflex DAC reference design use the links below.
Reflex DAC - 14 Bit 2MSPS +/- 10V Range
Reflex DAC - 14 Bit 2MSPS +/- 4.096V Range
Anabit's DAC Forum for comments and questions from this tutorial