RHF1401 datasheet(17/29 Pages) STMICROELECTRONICS

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RHF1401

Application information

17/29

The magnitude of the analog input common mode, CM should stay close to VREFP/2. Higher

levels will introduce more distortion.

7.1.2

Driving the analog inputs

The RHF1401 is designed to be differentially driven for better noise immunity.

Measurements done with single-ended signals show reduced levels of performance.

The switch-capacitor input structure of RHF1401 has a high input impedance (3.3 k

Ω at

FS = 20MHz) but it is not constant in time (see the equivalent input circuit in Figure 20)

because, at the end of each conversion, the charge update of the sampling capacitor draws

or injects a small transient current on the input signal.

One method of masking this transient current is a low-pass RC filter as shown in Figure 21

and Figure 22. A capacitor with a higher value than the sampling capacitor of 2.4 pF,

mounted in parallel with the two analog input signals, will absorb the transient glitches.

Figure 20.

ADC input equivalent circuit

Figure 18.

Linearity vs. Fs

(1)

Figure 19.

Distortion vs. Fs

(1)

At Fin = 5 MHz, using external REFP = 1.25 V, Icca optimized, VINCM = 0.65 V

Fs (Mhz)

11.1

11.2

11.3

11.4

11.5

11.6

11.7

11.8

11.9

12.1

ENOB

SINAD

SNR

-100

-95

-90

-85

-80

-75

-70

3579

Fs (Mhz)

THD

SFDR

Cs=2.4pF

VIN

AVcc

AGND

Switch at Fs

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