Texas Instruments SLAU039, Руководство пользователя

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Analog Input
2-6
2.6 Analog Input
The AD50 can be operated from a single 5 V supply or from a 5 V analog supply
and a 3.3 V digital supply. The AD50-EVM uses 5 V for both analog and digital
supplies to be compatible with the DSK+, and generates an additional –5 V
supply on the board. This negative supply allows the input and output circuitry
to be dc coupled. In an ac coupled system alternative configurations may be
used that do not require a negative supply.
The input circuitry consists of a switchable gain preamp, a single-ended to dif-
ferential converter with level shifting, and an antialiasing filter.
2.6.1 Preamp Design
This amplifier stage, illustrated in Figure 2–2 allows the use of either a line level
input (approximately 1 Vrms full scale) or a low level input (approximately
100mVrms full scale). With the link labeled
GAIN removed the preamp has uni-
ty gain from dc to 20 kHz. With this link inserted the preamp gain is increased
by 20 dB.
The signal can be ac coupled, by removing the link labeled DC. The use of two
tantalum capacitors back-to-back allows
±
10 V dc voltage bias at the input
without damaging the capacitors.
Figure 2–2. AD50-EVM Input Preamp
_
+ 1/2 TLC2272
5 V
–5 V
100 nF
100 nF
22 k
Ω
1 nF
2.2 k
Ω
Gain
4.7
µ
F
Tant
4.7
µ
F
Tant
22 k
Ω
DC
IN
This preamp uses the op amp in the inverting configuration to ensure that the
input common mode range of the op amp cannot be exceeded, since both in-
puts are always at a voltage close to ground. In the non-inverting configuration
the op amp inputs can approach the supply rails, causing distortion.
2.6.2 Input Stage Design
The AD50 analog to digital section uses differential inputs biased at 2.5 V. Max-
imum code is generated with 3 V
PEAK-PEAK
on both differential inputs. The in-
put stage provides the necessary level shifting and single-ended to differential
conversion, prior to the inputs of the AD50.