TSA1204IF datasheet(14/20 Pages) STMICROELECTRONICS

14 / 20 page

TSA1204

14/20

Figure 5 : DC-coupled 2Vpp differential analog

input

Clock input

The TSA1204 performance is very dependant on

your clock input accuracy, in terms of aperture

jitter; the use of low jitter crystal controlled

oscillator is recommended.

The duty cycle must be between 45% and 55%.

The clock power supplies must be separated from

the ADC output ones to avoid digital noise

modulation at the output.

It is recommended to always keep the circuit

clocked, even at the lowest specified sampling

frequency of 0.5Msps, before applying the supply

voltages.

Power consumption

So as to optimize both performance and power

consumption of the TSA1204 according the

sampling frequency, a resistor is placed between

IPOL and the analog Ground pins. Therefore, the

total dissipation is adjustable from 10Msps up to

20Msps.

The TSA1204 will combine highest performances

and lowest consumption at 20Msps when Rpol is

equal to 54k

Ω.

At lower sampling frequency range, this value of

resistor may be adjusted in order to decrease the

analog

current

without

any

degradation

dynamic performances.

The table below sums up the relevant data.

Figure 6 : Total power consumption optimization

depending on Rpol value

Layout precautions

To use the ADC circuits in the best manner at high

frequencies, some precautions have to be taken

for power supplies:

- First of all, the implementation of 4 separate

proper supplies and ground planes (analog,

digital, internal and external buffer ones) on the

PCB is recommended for high speed circuit

applications to provide low inductance and low

resistance common return.

The separation of the analog signal from the

digital part is mandatory to prevent noise from

coupling

onto

the

input

signal.

The

best

compromise is to connect from one part AGND,

DGND, GNDBI in a common point whereas

GNDBE must be isolated. Similarly, the power

supplies AVCC, DVCC and VCCBI must be

separated from the VCCBE one.

- Power supply bypass capacitors must be placed

as close as possible to the IC pins in order to

improve high frequency bypassing and reduce

harmonic distortion.

- Proper termination of all inputs and outputs must

be incorporated with output termination resistors;

then the amplifier load will be only resistive and

the stability of the amplifier will be improved. All

leads must be wide and as short as possible

especially for the analog input in order to decrease

parasitic capacitance and inductance.

- To keep the capacitive loading as low as

possible at digital outputs, short lead lengths of

routing are essential to minimize currents when

the output changes. To minimize this output

capacitance, buffers or latches close to the output

pins will relax this constraint.

- Choose component sizes as small as possible

(SMD).

APPLICATION

Digital Interface application

Thanks to its wide external buffer power supply

range, the TSA1204 is perfectly suitable to plug in

to 2.5V low voltage DSPs or digital interfaces as

well as to 3.3V ones.

Medical Imaging application

Driven by the demand of the applications requiring

nowadays either portability or high degree of par-

allelism (or both), this product has been devel-

oped to satisfy medical imaging, and telecom in-

frastructures needs.

As a typical system diagram shows figure 7, a nar-

row input beam of acoustic energy is sent into a

living body via the transducer and the energy re-

flected back is analyzed.

Fs (Msps)

Rpol (

Ω)

120

Optimized

power (mW)

120

TSA1204

VIN

VINB

INCM

330pF

4.7uF

10nF

analog

AC+DC

VREFP

VREFM

analog

VREFP-VREFM = 1 V

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