AFS090 datasheet(116/334 Pages) MICROSEMI

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Device Architecture

2- 10 0

R e v isio n 2

There are several popular ADC architectures, each with advantages and limitations. The analog-to-digital

converter in Fusion devices is a switched-capacitor Successive Approximation Register (SAR) ADC. It

supports 8-, 10-, and 12-bit modes of operation with a cumulative sample rate up to 600 k samples per

second (ksps). Built-in bandgap circuitry offers 1% internal voltage reference accuracy or an external

reference voltage can be used.

As shown in Figure 2-79, a SAR ADC contains N capacitors with binary-weighted values.

To begin a conversion, all of the capacitors are quickly discharged. Then VIN is applied to all the

capacitors for a period of time (acquisition time) during which the capacitors are charged to a value very

close to VIN. Then all of the capacitors are switched to ground, and thus –VIN is applied across the

comparator. Now the conversion process begins. First, C is switched to VREF. Because of the binary

weighting of the capacitors, the voltage at the input of the comparator is then shown by EQ 11.

Voltage at input of comparator = –VIN + VREF / 2

EQ 11

If VIN is greater than VREF / 2, the output of the comparator is 1; otherwise, the comparator output is 0.

A register is clocked to retain this value as the MSB of the result. Next, if the MSB is 0, C is switched

back to ground; otherwise, it remains connected to VREF, and C / 2 is connected to VREF. The result at

the comparator input is now either –VIN + VREF / 4 or –VIN + 3 VREF / 4 (depending on the state of the

MSB), and the comparator output now indicates the value of the next most significant bit. This bit is

likewise registered, and the process continues for each subsequent bit until a conversion is completed.

The conversion process requires some acquisition time plus N + 1 ADC clock cycles to complete.

Figure 2-79 • Example SAR ADC Architecture

Comparator

C / 2

C / 4

C / 2N–2

C / 2N–1

VREF

VIN

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