TSA1002IFT datasheet(14/20 Pages) STMICROELECTRONICS

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TSA1002

14/20

DRIVING THE ANALOG INPUT

Differential inputs

The TSA1002 has been designed to obtain

optimum performances when being differentially

driven. An RF transformer is a good way to

achieve such performances.

Figure 3 describes the schematics. The input

signal is fed to the primary of the transformer,

while the secondary drives both ADC inputs.

Figure 3 : Differential input configuration with

transformer

The common mode voltage of the ADC (INCM) is

connected to the center-tap of the secondary of

the transformer in order to bias the input signal

around this common voltage, internally set to

0.57V. The INCM is decoupled to maintain a low

noise level on this node. Our evaluation board is

mounted with a 1:1 ADT1-1WT transformer from

Minicircuits.

You

might

also

use

higher

impedance ratio (1:2 or 1:4) to reduce the driving

requirement on the analog signal source. For

example, with internal references, each analog

input can drive a 1Vpp amplitude input signal, so

the resultant differential amplitude is 2Vpp.

Figure 4 : AC-coupled differential input

Figure 4 represents the biasing of a differential

input signal in AC-coupled differential input

configuration. Both inputs VIN and VINB are

centered around the common mode voltage, that

can be let internal or fixed externally.

Figure 5 shows a DC-coupled configuration with

forced INCM to the DC analog input (mid-voltage)

while VREFM is connected to ground and VREFP

is let internal (1V); we achieve a 2Vpp differential

amplitude.

Figure 5 : DC-coupled 2Vpp differential analog

input

Single-ended input configuration

The single-ended input configuration of the

TSA1002 requires particular biasing and driving.

The structure being fully differential, care has to

be taken in order to properly bias the inputs in sin-

gle ended mode. Figure 6 summarizes the link

from the differential configuration to the sin-

gle-ended one; a wrong configuration is also pre-

sented.

- With differential driving, both inputs are centered

around the INCM voltage.

- The transition to single-ended configuration

implies to connect the unused input (VINB for

instance) to the DC component of the single input

(Vin) and also to the input common mode in order

to be well balanced. The mid-code is achieved at

the crossing between VIN and VINB, therefore

inputs are conveniently biased.

- Unlikely other structures of converters in which

the unused input can be grounded; in our case it

will end with unbalanced inputs and saturation of

the internal amplifiers leading to a non respect of

the output codes.

TSA1002

VIN

VINB

INCM

Ω

100pF

330pF

4.7uF

10nF

Analog source

1:1

ADT1-1

Ω

10nF

TSA1002

VIN

VINB

INCM

33pF

100k

Ω

100k

Ω

10nF

common

mode

TSA1002

VIN

VINB

INCM

330pF

4.7uF

10nF

analog

AC+DC

VREFP

VREFM

VREFP-VREFM = 1 V

analog

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