AD12400KWS datasheet(15/28 Pages) AD

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AD12400

Rev. 0 | Page 15 of 28

THEORY OF OPERATION

The AD12400 uses two high-speed, 12-bit analog-to-digital

converters (ADCs) in a time-interleaved configuration to

double the sample rate, while maintaining a high level of

dynamic range performance. The digital output of each ADC

channel is calibrated using a proprietary digital post processing

technique, Advanced Filter Bank (AFBTM), from VCorp

Technologies. AFB is implemented using a state-of-the-art Field

Programmable Gate Array (FPGA) and provides a wide

bandwidth, wide temperature match for any gain, phase, and

clock timing errors between each ADC channel.

TIME-INTERLEAVING ADCS

When two ADCs are time-interleaved, gain and/or phase

mismatches between each channel will produce an

Image Spur

at fs/2 – fAIN and an Offset Spur as shown in Figure 18. These

mismatches can be the result of any combination of device

tolerance, temperature, and frequency deviations.

–120

–110

–100

–90

–80

–70

–60

–50

–40

–30

–20

–10

100

120

140

160

180

200

FREQUENCY (MHz)

IMAGE SPUR

OFFSET SPUR

Figure 18. Image Spur Due to Mismatches between Two Interleaved ADCs

(No AFB Digital Post Processing)

Figure 19 displays the performance of a similar converter with

on-board, AFB post processing implemented. The –44 dBFS

image spur has been reduced to –77 dBFS and as a result, the

dynamic range of this time-interleaved ADC is no longer

limited by the channel matching.

100

120

140

160

180

200

FREQUENCY (MHz)

–120

–110

–100

–90

–80

–70

–60

–50

–40

–30

–20

–10

IMAGE SPUR

OFFSET SPUR

Figure 19. AD12400 with AFB Digital Post Processing

The relationship between image spur and channel mismatches

is captured in Table 7 for specific conditions.

Table 7. Image Spur vs. Channel Mismatch

Gain Error

(%)

Aperture Delay Error

(ps)

Image Spur

(dBc)

–40

0.25

2.7

–54

0.2

1.1

–62

0.025

0.5

–70

For a more detailed description of time-interleaving in ADCs

and a design example using the AD12400, refer to “Advanced

Digital Post-Processing Techniques Enhance Performance in

Time Interleaved ADC Systems,” published in the August, 2003

edition of Analog Dialogue. This article can be found at

http://www.analog.com/analogDialogue.

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