TSA1401 datasheet(13/19 Pages) STMICROELECTRONICS

13 / 19 page

APPLICATION INFORMATION

TSA1401

13/19

Fig. 11: Distortion vs. VREFP

Fin=5MHz;Fs=20Mhz;Icca=26mA;INCM=0.46V

An external reference voltage device may be used

for specific applications requiring even better

linearity, accuracy or enhanced temperature

behavior.

Using the STMicroelectronics TS821, TS4041-1.2

or TS431 Voltage Reference devices leads to

optimum

performances

when

configured

shown in

Figure 12. The full scale is increased to

2.5Vpp differential and SNR and SINAD are

enhanced as shown in

Figure 13 .

Fig. 12: External reference setting

In multi-channel applications, the high impedance

input of the references permits to drive several

ADCs with only one Voltage Reference device.

Fig. 13: Linearity vs. Fs at Fin=5MHz, using

TS4041 Icca optimised; VREFP=1.225V;

VREFM=GND; INCM=0.65V,

Fig. 14: Distortion vs. Fs at Fin=5MHz, using

TS4041 Icca optimised; VREFP=1.225V;

VREFM=GND; INCM=0.65V

The magnitude of the analog input common

mode, CM should stay close to VREFP/2. Higher

level will introduce more distortion.

5.1.2 - Driving the analog input

The

TSA1401

has

been

designed

differentially driven for better noise immunity.

Some measurements have been done with

single-ended signals. It degrades a little bit the

performances, with an SFDR of -75dBc and an

ENOB of 11.2 bits at 20Msps, Fin at 10MHz.

The switch-capacitor input structure of TSA1401,

presents a high input impedance (3.3k

Ω at

Fs=20MHz)

but

not

constant

time

(see

equivalent input circuit

Figure 15). Indeed at the

end of each conversion, the charge update of the

-100

-95

-90

-85

-80

-75

-70

0.8

0.9

1.1

1.2

1.3

1.4

REFP (V)

THD

SFDR

Ω

TSA1401

VIN

VINB

VREFM

VREFP

external

reference

AVCC

330pF

4.7µF

10nF

TS821

TS4041

REFMODE

100

Ω

3579

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

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