MOTOROLA
Chapter 14. QADC64E Enhanced Mode Operation
14-77
QADC64E Integration Requirements
14.7.4
Analog Reference Signals
VRH and VRL are the dedicated input signals for the high and low reference voltages.
Separating the reference inputs from the power supply signals allows for additional external
filtering, which increases reference voltage precision and stability, and subsequently
contributes to a higher degree of conversion accuracy.
The AltRef signal may be selected through the CCW as the high reference for a conversion.
This allows for the ability to “zoom” in on a portion of the convertible range with the full
No A/D converter can be more accurate than its analog reference. Any noise in the
reference can result in at least that much error in a conversion. The reference for the
QADC64E, supplied by signals V
RH, AltRef, and VRL, should be low-pass filtered from its
source to obtain a noise-free, clean signal. In many cases, simple capacitive bypassing may
sufficed. In extreme cases, inductors or ferrite beads may be necessary if noise or RF energy
is present. Series resistance is not advisable since there is an effective DC current
requirement from the reference voltage by the internal resistor string in the RC DAC array.
External resistance may introduce error in this architecture under certain conditions. Any
series devices in the filter network should contain a minimum amount of DC resistance.
14.7.5
Analog Input Signals
Analog inputs should have low AC impedance at the signals. Low AC impedance can be
realized by placing a capacitor with good high frequency characteristics at the input signal
of the part. Ideally, that capacitor should be as large as possible (within the practical range
of capacitors that still have good high frequency characteristics). This capacitor has two
effects:
It helps attenuate any noise that may exist on the input.
It sources charge during the sample period when the analog signal source is a
high-impedance source.
Series resistance can be used with the capacitor on an input signal to implement a simple
RC filter. The maximum level of filtering at the input signals is application dependent and
is based on the bandpass characteristics required to accurately track the dynamic
characteristics of an input. Simple RC filtering at the signal may be limited by the source
impedance of the transducer or circuit supplying the analog signal to be measured. Refer to
size of the capacitor at the signal may be very small.
Figure 14-50 is a simplified model of an input channel. Refer to this model in the following
discussion of the interaction between the external circuitry and the circuitry inside the
QADC64E.