EVAL1002/AA View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'EVAL1002/AA' PDF : 20 Pages View PDF
TSA1002
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
Figure 4 represents the biasing of a differential
N 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.
G Figure 5 shows a DC-coupled configuration with
I forced INCM to the DC analog input (mid-voltage)
while VREFM is connected to ground and VREFP
S is let internal (1V); we achieve a 2Vpp differential
E amplitude.
D Figure 5 : DC-coupled 2Vpp differential analog
input
Analog source
ADT1-1
1:1
E W t(s) 50Ω
100pF
VIN
TSA1002
VINB
INCM
R N roduc 330pF 10nF 4.7uF
analog
DC
analog
DC
VREFP-VREFM = 1 V
AC+DC
VREFP
VIN
TSA1002
VINB
VREFM
INCM
330pF 10nF 4.7uF
O lete P The common mode voltage of the ADC (INCM) is
F o connected to the center-tap of the secondary of
s the transformer in order to bias the input signal
b around this common voltage, internally set to
T O 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
O t(s Minicircuits. You might also use a higher
c impedance ratio (1:2 or 1:4) to reduce the driving
N u requirement on the analog signal source. For
d example, with internal references, each analog
ro input can drive a 1Vpp amplitude input signal, so
P the resultant differential amplitude is 2Vpp.
te Figure 4 : AC-coupled differential input
Obsole50Ω 10nF
33pF
100kΩ
VIN
INCM TSA1002
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.
common
mode
100kΩ
VINB
- Unlikely other structures of converters in which
the unused input can be grounded; in our case it
50Ω 10nF
will end with unbalanced inputs and saturation of
the internal amplifiers leading to a non respect of
the output codes.
14/20
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