ADA4355ABCZ datasheet(35/45 Pages) AD

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Data Sheet

ADA4355

Rev. A | Page 35 of 45

High Spatial Resolution

The ADA4355 has a maximum sampling rate of 125 MSPS,

which translates to an 8 ns sampling period and a best case

theoretical spatial resolution of approximately 0.8 m. This

resolution is marginally acceptable for pulse widths of

approximately 10 ns or greater. Although, with only one

sample per pulse, closely spaced events can be difficult to

detect. However, while using a fixed sampling rate, spatial

resolution can be increased by using a phase shift to advance

or delay the laser pulse and/or the rising and falling edge of the

sampling clock.

Figure 96 shows the sampling period of the ADA4355 and the

placement of the laser driving pulses. In this example, four

phase shifts were applied (0 ns, 2 ns, 4 ns, and 6 ns). This

phase shifting effectively improves the spatial resolution from

approximately 0.8 m to 0.2 m, thereby enabling the use of

shorter pulse widths and positive detection of more closely

spaced events.

LASER

DRIVING PULSE

ADC

SAMPLING EDGE

6ns

8ns

4ns

2ns

Figure 96. Phase Shifting the Laser Pulses

Figure 97 clearly shows the benefit of phase shifting for the

detection of closely spaced events. A 4 ns pulse was used to

detect two events 1 m apart. Figure 97 shows two distinct

events are detected in the phase shifted case, while the two

events blend together and appear as a single event in the

nonphase shifted case.

–5 –4 –3 –2 –1 0

9 10 11 12 13 14 15

DISTANCE (m)

–24

–22

–20

–18

–16

–14

–12

–10

–8

–6

–4

–2

4ns PHASE SHIFTED

4ns NONPHASE SHIFTED

Figure 97. Comparison of Closely Spaced Event Detection with and Without

Phase Shifting, 4 ns Pulse Width, 1 m Event Separation

Figure 98 illustrates the method used to build the phase shifted

curve from Figure 97. The nonphase shifted measurement was

repeated four times, but each time it was shifted an additional

2 ns to the right. The phase shifted curve from Figure 97 was

then assembled by taking the points from the four individual

measurements and combining these points as shown in Figure 98.

The end result takes four times longer to complete. However,

the end result has four times as many data points (2 ns apart),

resulting in much improved horizontal resolutions.

0ns PHASE SHIFT

...

2ns PHASE SHIFT

NONPHASE SHIFTED CURVES

PHASE SHIFTED CURVE (COMBINATION OF FOUR NONPHASE SHIFTED CURVES)

4ns PHASE SHIFT

...

6ns PHASE SHIFT

Figure 98. Method Used to Assemble Phase Shifted Curve From Figure 97

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