9.1 Analog and Digital Power Supplies

Any ADC architecture is sensitive to spikes on the power supply, reference, and ground pins. These spikes may originate from switching power supplies, digital logic, high power devices, and other sources. Power to the ADC161S626 should be clean and well bypassed. A 0.1 µF ceramic bypass capacitor and a 1 µF to 10 µF capacitor should be used to bypass the ADC161S626 supply, with the 0.1 µF capacitor placed as close to the ADC161S626 package as possible.

Since the ADC161S626 has both the V_A and V_IO pins, the user has three options on how to connect these pins. The first option is to tie V_A and V_IO together and power them with the same power supply. This is the most cost effective way of powering the ADC161S626 but is also the least ideal. As stated previously, noise from V_IO can couple into V_A and adversely affect performance. The other two options involve the user powering V_A and V_IO with separate supply voltages. These supply voltages can have the same amplitude or they can be different. V_A can be set to any value between +4.5V and +5.5V; while V_IO can be set to any value between +2.7V and +5.5V.

Best performance will typically be achieved with V_A operating at 5V and V_IO at 3V. Operating V_A at 5V offers the best linearity and dynamic performance when V_REF is also set to 5V; while operating V_IO at 3V reduces the power consumption of the digital logic. Operating the digital interface at 3V also has the added benefit of decreasing the noise created by charging and discharging the capacitance of the digital interface pins.

9.2 Voltage Reference

The reference source must have a low output impedance and needs to be bypassed with a minimum capacitor value of 0.1 µF. A larger capacitor value of 1 µF to 10 µF placed in parallel with the 0.1 µF is preferred. While the ADC161S626 draws very little current from the reference on average, there are higher instantaneous current spikes at the reference.

V_REF of the ADC161S626, like all A/D converters, does not reject noise or voltage variations. Keep this in mind if V_REF is derived from the power supply. Any noise and/or ripple from the supply that is not rejected by the external reference circuitry will appear in the digital results. The use of an active reference source is recommended. The LM4040 and LM4050 shunt reference families and the LM4120 and LM4140 series reference families are excellent choices for a reference source.