Functional Description

PQA4

PQA0

PQB3

PQB0

VRH

VRL

VDDA VSSA

 

16

 

 

 

 

 

 

 

Chan. Decode & MUX

4

 

 

CHAN[5:0]

 

 

 

 

 

 

 

 

16:1

 

 

 

 

 

 

6

 

 

 

 

 

 

 

 

 

 

 

10-bit A/D Converter

Input

Bias Circuit

 

 

 

 

 

 

 

 

 

 

 

 

Internal

Power-

 

STOP

Logic

 

 

Sample

Channel

Down

 

RST

 

 

Buffer

Decode

 

 

QCLK

Control

 

 

 

 

 

 

 

 

 

 

2

 

 

 

State Machine & Logic

IST

 

 

 

 

 

 

CSAMP

 

 

 

Start Conv

Queue

 

 

SAR Timing

 

End OF Conv

 

 

 

 

 

 

10

 

 

 

SAR[9:0]

From/to

 

 

 

 

 

 

Signals

 

 

 

10

10

 

 

 

 

 

 

 

 

 

Analog

Compar-

Successive

 

 

 

 

Approximation

 

 

 

Power

 

ator

Register

 

 

 

 

 

 

 

 

 

 

Figure 27-19. QADC Analog Subsystem Block Diagram

27.7.3.2Conversion Cycle Times

Total conversion time is made up of initial sample time, final sample time, and resolution time. Initial sample time refers to the time during which the selected input channel is coupled through the sample buffer amplifier to the sample capacitor. The sample buffer is used to quickly reproduce its input signal on the sample capacitor and minimize charge sharing errors. During the final sampling period the amplifier is bypassed, and the multiplexer input charges the sample capacitor array directly for improved accuracy. During the resolution period, the voltage in the sample capacitor is converted to a digital value and stored in the SAR as shown in Figure 27-20.

Initial sample time is fixed at two QCLK cycles. Final sample time can be 2, 4, 8, or 16 QCLK cycles, depending on the value of the IST field in the CCW. Resolution time is 10 QCLK cycles.

Aconversion requires a minimum of 14 QCLK cycles (7 ∝s with a 2.0-MHz QCLK). If the maximum final sample time period of 16 QCLKs is selected, the total conversion time is 28 QCLKs or 14 ∝s (with a 2.0-MHz QCLK).

MOTOROLA

Chapter 27. Queued Analog-to-Digital Converter (QADC)

27-35

Page 619
Image 619
Motorola MCF5281, MCF5282 user manual Conversion Cycle Times, Qadc Analog Subsystem Block Diagram