HMCAD1520 Datasheet, PDF (33/36 Pages) Hittite Microwave Corporation – High Speed Multi-Mode 8/12/14-Bit 1000/640/105 MSPS A/D Converter

v03.0711
HMCAD1520
High Speed Multi-Mode 8/12/14-Bit
1000/640/105 MSPS A/D Converter
Table 28: Overview of Fine
Gain Usage in Precision Mode
Channel #
1
2
3
4
LVDS output
D1A, (D1B)
D2A, (D2B)
D3A, (D3B)
D4A, (D4B)
Fine gain branch
1
3
5
7
Input
Input
Amplifier
43 Ω
33 pF
43 Ω
CM_EXT
IPx
INx
Analog Input
The analog input to HMCAD1520 is a switched capacitor
Figure 14: DC coupled input
track-and-hold amplifier optimized for differential
operation. Operation at common mode voltages at mid
The input amplifier could be inside a companion chip
supply is recommended even if performance will be
or it could be a dedicated amplifier. Several suitable
0
good for the ranges specified. The VCM pin provides
a voltage suitable as common mode voltage reference.
single ended to differential driver amplifiers exist in the
market. The system designer should make sure the
The internal buffer for the VCM voltage can be
specifications of the selected amplifier is adequate for
switched off, and driving capabilities can be changed
the total system, and that driving capabilities comply
programming the ext_vcm_bc<1:0> register.
with HMCAD1520 input specifications.
track
IPx
hold
track
track
track
INx
hold
Figure 13: Input configuration
Figure 13 shows a simplified drawing of the input
network. The signal source must have sufficiently low
output impedance to charge the sampling capacitors
within one clock cycle. A small external resistor (e.g.
22 ohm) in series with each input is recommended as it
helps reducing transient currents and dampens ringing
behavior. A small differential shunt capacitor at the chip
side of the resistors may be used to provide dynamic
charging currents and may improve performance. The
resistors form a low pass filter with the capacitor, and
values must therefore be determined by requirements
for the application.
DC-Coupling
Figure 14 shows a recommended configuration for
DC-coupling. Note that the common mode input
voltage must be controlled according to specified
values. Preferably, the CM_EXT output should be
used as reference to set the common mode voltage.
Detailed configuration and usage instructions must be
found in the documentation of the selected driver, and
the values given in figure 14 must be varied according
to the recommendations for the driver.
AC-Coupling
Input
33 Ω
R_T
47Ω
33 Ω
CM_EXT
IPx
INx
Figure 15: Transformer coupled input
A signal transformer or series capacitors can be
used to make an AC-coupled input network. Figure
15 shows a recommended configuration using a
transformer. Make sure that a transformer with
sufficient linearity is selected, and that the bandwidth
of the transformer is appropriate. The bandwidth
should exceed the sampling rate of the ADC with at
least a factor of 10. It is also important to minimize
phase mismatch between the differential ADC inputs
for good HD2 performance. This type of transformer
coupled input is the preferred configuration for high
frequency signals as most differential amplifiers do
not have adequate performance at high frequencies.
0 - 33
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