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HMCAD1050-40 Datasheet(PDF) 9 Page - Hittite Microwave Corporation

Part No. HMCAD1050-40
Description  Dual 13/12-Bit 20/40 MSPS A/D Converter
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Maker  HITTITE [Hittite Microwave Corporation]
Homepage  http://www.hittite.com
Logo HITTITE - Hittite Microwave Corporation

HMCAD1050-40 Datasheet(HTML) 9 Page - Hittite Microwave Corporation

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For price, delivery and to place orders: Hittite Microwave Corporation, 2 Elizabeth Drive, Chelmsford, MA 01824
978-250-3343 tel • 978-250-3373 fax • Order On-line at www.hittite.com
Application Support: apps@hittite.com
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Dual 13/12-Bit 20/40 MSPS
a/D Converter
recommended usage
Analog Input
the analog inputs to the HMCAD1050-40 is a switched
capacitor track-and-hold amplifier optimized for differ-
ential operation. operation at common mode voltages
at mid supply is recommended even if performance
will be good for the ranges specified. the CM_eXt pin
provides a voltage suitable as common mode voltage
reference. the internal buffer for the CM_eXt voltage
can be switched off, and driving capabilities can be
changed by using the CM_eXtBC control input.
Figure 4 shows a simplified drawing of the input net-
work. 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 perfor-
mance. the resistors form a low pass filter with the
capacitor, and values must therefore be determined by
requirements for the application.
Figure 4: Input configuration
Figure 5 shows a recommended configuration for DC-
coupling. note that the common mode input voltage
must be controlled according to specified values. Pref-
erably, the CM_eXt output should be used as refer-
ence to set the common mode voltage.
Figure 5: DC coupled input with buffer
the input amplifier could be inside a companion chip
or it could be a dedicated amplifier. several suitable
single ended to differential driver amplifiers exist in the
market. the system designer should make sure the
specifications of the selected amplifier is adequate for
the total system, and that driving capabilities comply
with the HMCAD1050-40 input specifications.
Detailed configuration and usage instructions must be
found in the documentation of the selected driver, and
the values given in figure 5 must be varied according
to the recommendations for the driver.
A signal transformer or series capacitors can be used
to make an AC-coupled input network. Figure 6 shows
a recommended configuration using a transformer.
Figure 6: transformer coupled input
Make sure that a transformer with sufficient linearity is
selected, and that the bandwidth of the transformer is
appropriate. the bandwidth should exceed the sam-
pling 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 dif-
ferential amplifiers do not have adequate performance
at high frequencies. Magnetic coupling between
the transformers and PCB traces may impact chan-
nel crosstalk, and must hence be taken into account
during PCB layout. If the input signal is traveling a long
physical distance from the signal source to the trans-
former (for example a long cable), kick-backs from the
ADC will also travel along this distance. If these kick-
backs are not terminated properly at the source side,
they are reflected and will add to the input signal at the
ADC input. this could reduce the ADC performance.
to avoid this effect, the source must effectively ter-
minate the ADC kick-backs, or the traveling distance
should be very short. If this problem could not be
avoided, the circuit in figure 8 can be used.
Figure 7 shows AC-coupling using capacitors. resis-
tors from the CM_eXt output, rCM, should be used
to bias the differential input signals to the correct volt-

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