TSA1204IFT datasheet(13/20 Pages) STMICROELECTRONICS

13 / 20 page

TSA1204

13/20

External reference and common mode

Each of the voltages VREFM, VREFP and INCM

can be fixed externally to better fit to the

application needs (Refer to table’ OPERATING

CONDITIONS’ page 4 for min/max values).

The VREFP, VREFM voltages set the analog

dynamic at the input of the converter that has a full

scale amplitude of 2*(VREFP-VREFM). Using

internal VREFP, the dynamic range is 1.8V.

The best linearity and distortion performances are

achieved with a dynamic range above 2Vpp and

by increasing the VREFM voltage instead of

lowering the VREFP one.

The INCM is the mid voltage of the analog input

signal.

It is possible to use an external reference voltage

device for specific applications requiring even

better

linearity,

accuracy

enhanced

temperature behavior.

Using

the

STMicroelectronics

TS821or

TS4041-1.2 Vref leads to optimum performances

when configured as shown on Figure 2.

Figure 2 : External reference setting

DRIVING THE DIFFERENTIAL ANALOG

INPUTS

The TSA1204 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. 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

0.46V. It determines the DC component of the

analog signal. As being an high impedance input,

it acts as an I/O and can be externally driven to

adjust

this

component.

The

INCM

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 a higher impedance ratio (1:2 or

1:4) to reduce the driving requirement on the

analog signal source.

Each analog input can drive a 1.4Vpp amplitude

input signal, so the resultant differential amplitude

is 2.8Vpp.

Figure 3 : Differential input configuration with

transformer

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 4 : AC-coupled differential input

Figure 5 shows a DC-coupled configuration with

forced VREFP and INCM to the 1V DC analog

input while VREFM is connected to ground; we

achieve a 2Vpp differential amplitude.

Ω

TSA1204

VIN

VINB

VREFM

VREFP

external

reference

VCCA

330pF

4.7uF

10nF

TS821

TS4041

TSA1204

VIN

VINB

INCM

Ω

33pF

330pF

470nF

10nF

Analog source

1:1

ADT1-1

I or Q ch.

Ω

10nF

TSA1204

VIN

VINB

INCM

33pF

100k

Ω

100k

Ω

10nF

common

mode

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