7.3.2 Reference Pins

The ADC14155QML-SP is designed to operate with an internal 1-V reference, or an external 1-V reference, but performs well with external reference voltages in the range of 0.9 V to 1.1 V. The internal 1-V reference is the default condition when no external reference input is applied to the V_REF pin. If a voltage in the range of 0.9 V to 1.1 V is applied to the V_REF pin, then that voltage is used for the reference. The V_REF pin should always be bypassed to ground with a 0.1-µF capacitor close to the reference input pin. Lower reference voltages will decrease the signal-to-noise ratio (SNR) of the ADC14155. Increasing the reference voltage (and the input signal swing) beyond 1.1-V may degrade THD for a full-scale input, especially at higher input frequencies.

It is important that all grounds associated with the reference voltage and the analog input signal make connection to the ground plane at a single, quiet point to minimize the effects of noise currents in the ground path.

The Reference Bypass Pins (V_RP, V_RM, and V_RN) are made available for bypass purposes. Each of these pins should be bypassed to ground with a 0.1-µF capacitor. A 0.1-µF and a 10-µF capacitor should be placed between the V_RP and V_RN pins, as shown in Figure 24. This configuration is necessary to avoid reference oscillation, which could result in reduced SFDR and/or SNR. V_RM may be loaded to 1 mA for use as a temperature stable 1.5-V reference. The  V_RP and  V_RN pins should not be loaded.

Smaller capacitor values than those specified will allow faster recovery from the power down mode, but may result in degraded noise performance. Loading any of these pins, other than V_RM, may result in performance degradation.

The nominal voltages for the reference bypass pins are as follows:

 V_RM = 1.5 V

 V_RP = V_RM + V_REF / 2

 V_RN = V_RM – V_REF / 2