28.2.1 Module Description

The CBG module consists of two classes of current sources: four 10 µA sources and four selectable sources ranging from 30-200 µA. Both sets of current sources are routed to the same output pin.

Both sets of current sources can be used in general current sourcing applications to generate voltages using an external resistor or to provide biasing to external circuitry or sensors.

One intended use of the selectable current source is to generate an offset voltage to shift a positive external signal to be within the input range of the internal analog peripherals, such as the ADC. Shifting the input voltage maintains the dynamic range of the AC component of the input signal but removes the offset voltage. An external resistor (refer to Figure 28-2) is used in conjunction with the current source to develop the offset voltage.

The external resistors are large due to the small generated currents. This large resistor value protects the device input circuitry by limiting the current injected into the device when the current source is not enabled.

Both classes of current sources can be externally paralleled by connecting the output pins together to increase current.

The large resistors used to create the voltage offset may exceed the ADC input impedance specification. To meet the ADC input requirements, one or more of the following may be required:
  • Increase in sampling time
  • Use of an internal amplifier, such as an op amp or PGA
  • Use of a small capacitor on the input pin if the input signal does not change quickly
  • Use of an AC bypass capacitor

A high-level diagram of the CBG circuitry is shown in Figure 28-1.

Figure 28-1. Current Bias Generator Sources
Note:
  1. RESD is typically 300 Ω.