EVAL1203/BA View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'EVAL1203/BA' PDF : 30 Pages View PDF
TSA1203
External reference and common mode
Each of the voltages VREFP and INCM can be
fixed externally to better fit to the application
needs (Refer to Table ’OPERATING
CONDITIONS’ page 5 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 references, 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 or enhanced
temperature behavior.
Using the STMicroelectronics TS821 or
TS4041-1.2 Vref leads to optimum performances
when configured as shown on Figure 2 .
Figure 2 : External reference setting
analog signal. As being an high impedance input,
it acts as an I/O and can be externally driven to
adjust this DC component. 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 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 1Vpp amplitude in-
put signal, so the resultant differential amplitude is
2Vpp.
Figure 3 : Differential input configuration with
transformer
Analog source
ADT1-1
1:1
VIN
TSA1203
50Ω
33pF
I or Q ch.
VINB
INCM
330pF
10nF 470nF
1kΩ
VCCA VREFP
VIN
TSA1203
VINB
VREFM
330pF 10nF 4.7uF
TS821
TS4041
external
reference
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.
DRIVING THE ANALOG INPUT
Differential inputs
The TSA1203 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
Figure 4 : AC-coupled differential input
50Ω
common
mode
10nF
33pF
50Ω 10nF
100kΩ
100kΩ
VIN
INCM TSA1203
VINB
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.
13/21
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