Design a Unipolar to Bipolar Converter for a Unipolar Voltage Output DAC

Unipolar to bipolar converters are useful when we have to have a unipolar component to do a certain job in a mixed signal design environment.  For example, Digital to Analog Converters (DACs) may have the output voltage range 0 to 2.5 V, or 0 to 5 V, while the design asks for a range of –5 V to +5 V.  To comply with this requirement, we have to design a unipolar to bipolar converter which will be inserted between the DAC output and the following bipolar stage.  It looks like the circuit in Figure 1.  How did I design it?

unipolar_to_bipolar_converter_1Figure 1

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Design a Bipolar to Unipolar Converter to Drive an ADC

Most Analog to Digital Converters have a unipolar input that can be a problem when designing bipolar circuits.  Some common ADC input voltage ranges are 0 to 2.5 V, or 0 to 5 V.  However, the analog circuit that drives the ADC can have voltage swings of, –1 V to +1 V, –2 V to +2 V , –5 V to +5 V, and so on.  Bringing the ADC input below ground is a big No-No, because the current from input will flow through the chip substrate creating irreversible changes in the ADC and damage it.  So, how do we connect a bipolar front end circuit with a unipolar ADC?  Enters the bipolar to unipolar converter.  Let’s design one.

The converter can be designed with a summing amplifier, as in Figure 1.  How do we calculate the resistors?

summing_amplifier1

Figure 1

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