ADS5400IPZP datasheet(32/48 Pages) TI

32 / 48 page

ADS5400

AIN

100 W

ACSignal

Source

1:1

Analog Input Amplitude − dBFS

−25

−20

−15

−10

−5

−1

G023

After Positive

Over-range

1GSPS (1ns)

After Negative

Over-range

1GSPS (1ns)

After Positive

Over-range

400MSPS (2.5ns)

After Positive

Over-range

200MSPS (5ns)

After Negative

Over-range

400MSPS (2.5ns)

After Negative

Over-range

200MSPS (5ns)

ADS5400

SLAS611B – OCT 2009 – REVISED MARCH 2010

www.ti.com

The ADS5400 obtains optimum performance when the analog inputs are driven differentially. The circuit in

Figure 28 shows one possible configuration using an RF transformer. Datasheet performance, especially at

>1GHz input frequency, can only be obtained with a carefully designed differential drive path to the ADC.

Figure 28. Converting a Single-Ended Input to a Differential Signal Using an RF Transformer

Voltage Reference

The 2V voltage reference is provided internal to the ADS5400. A VCM (voltage common mode) pin is provided

as an output for use in dc-coupled applications, equal to the AVDD5 supply divided by 2. This provides the

analog input common mode voltage to a driving circuit so that the common mode is setup properly. Some

systems may prefer the use of an external voltage reference. This mode can be enabled by pulling the

ENEXTREF pin high. In this mode, an external reference can be driven onto the VREF pin, which is normally

expecting 2V.

Analog Input Over-Range Recovery Error

An over-range condition occurs if the analog input voltage exceeds the full-scale range of the converter (0dBFS).

To test recovery from an over-range, the ADC analog input is injected with a sinusoidal input frequency exactly at

CLKIN/4 (a four-point sinusoid at the digital outputs). The four sample points of each period occur at the top,

mid-scale, bottom and mid-scale of the sinusoid (clipped by the ADC when over-ranged to all 0s or all 1s). Once

the amplitude exceeds 0dBFS, the top and bottom of the sinusoidal input becomes out of range, while the

mid-scale point is always in-range and measureable with ADC output codes. The graph in Figure 29 indicates the

amount of error from the expected mid-scale value of 2048 that occurs after negative over-range (bottom of

sinusoid) and positive over-range (top of sinusoid). This equates to the amount of error in a valid sample 1 clock

cycle after an over-range occurs, as a function of input amplitude.

Figure 29. Recovery Error 1 Clock Cycle After Over-Range vs Input Amplitude

Product Folder Link(s): ADS5400

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