000...001

000...010

0V

011...111

111...000

000...000

111...111

111...110

ANALOG INPUT

1 LSB = VA/4096

0.5 LSB

+VA-1.5 LSB

|

ADC121S051

www.ti.com

SNAS302I – JULY 2005 – REVISED JANUARY 2014

Sixteen SCLK cycles are required to read a complete sample from the ADC. The sample bits (including leading

zeroes) are clocked out on falling edges of SCLK, and are intended to be clocked in by a receiver on subsequent

falling edges of SCLK. The ADC will produce three leading zero bits on SDATA, followed by twelve data bits,

most significant first.

If CS goes low before the rising edge of SCLK, an additional (fourth) zero bit may be captured by the next falling

edge of SCLK.

Determining Throughput

Throughput depends on the frequency of SCLK and how much time is allowed to elapse between the end of one

conversion and the start of another. At the maximum specified SCLK frequency, the maximum ensured

throughput is obtained by using a 20 SCLK frame. As shown in Figure 4, the minimum allowed time between CS

falling edges is determined by 1) 12.5 SCLKs for Hold mode, 2) the larger of two quantities: either the minimum

required time for Track mode (tACQ) or 2.5 SCLKs to finish reading the result and 3) 0, 1/2 or 1 SCLK padding to

ensure an even number of SCLK cycles so there is a falling SCLK edge when CS next falls. For example, at the

fastest rate for this family of parts, SCLK is 20 MHz and 2.5 SCLKs are 125 ns, so the minimum time between

CS falling edges is calculated by:

12.5*50ns + 350ns + 0.5*50ns = 1000ns

(2)

(12.5 SCLKs + tACQ + 1/2 SCLK) which corresponds to a maximum throughput of 1MSPS. At the slowest rate for

this family, SCLK is 1MHz. Using a 20 cycle conversion frame as shown in Figure 4 yields a 20 µs time between

CS falling edges for a throughput of 50 KSPS. It is possible, however, to use fewer than 20 clock cycles provided

the timing parameters are met. With a 1MHz SCLK, there are 2500 ns in 2.5 SCLK cycles, which is greater than

tACQ. After the last data bit has come out, the clock will need one full cycle to return to a falling edge. Thus the

total time between falling edges of CS is 12.5*1µs +2.5*1µs +1*1µs=16

μs which is a throughput of 62.5 KSPS.

ADC121S051 TRANSFER FUNCTION

The output format of the ADC is straight binary. Code transitions occur midway between successive integer LSB

values. The LSB width for the ADC is VA/4096. The ideal transfer characteristic is shown in Figure 20. The

transition from an output code of 0000 0000 0000 to a code of 0000 0000 0001 is at 1/2 LSB, or a voltage of

VA/8192. Other code transitions occur at steps of one LSB.

Figure 20. Ideal Transfer Characteristic

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