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A / D CONVERTERS - SMT
0
0 - 9
HMCAD1041-40
v01.0411
SINGLE 10-BIT 20/40 MSPS A/D CONVERTER
Recommended Usage
Analog Input
The analog inputs to the HMCAD1041-40 is a switched
capacitor track-and-hold amplier optimized for differ-
ential operation. Operation at common mode voltages
at mid supply is recommended even if performance
will be good for the ranges specied. 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 simplied 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 lter with the
capacitor, and values must therefore be determined by
requirements for the application.
Figure 4: Input conguration
DC-Coupling
Figure 5 shows a recommended conguration for DC-
coupling. Note that the common mode input voltage
must be controlled according to specied 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 amplier could be inside a companion chip
or it could be a dedicated amplier. Several suitable
single ended to differential driver ampliers exist in the
market. The system designer should make sure the
specications of the selected amplier is adequate for
the total system, and that driving capabilities comply
with the HMCAD1041-40 input specications.
Detailed conguration and usage instructions must be
found in the documentation of the selected driver, and
the values given in gure 5 must be varied according
to the recommendations for the driver.
AC-Coupling
A signal transformer or series capacitors can be used
to make an AC-coupled input network. Figure 6 shows
a recommended conguration 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 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
conguration for high frequency signals as most differ-
ential ampliers do not have adequate performance at
high frequencies. 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 reected 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 gure 8 can be used.
Information furnished by Analog Devices is believed to be accurate and reliable. However, no
responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other
rights of third parties that may result from its use. Specifications subject to change without notice. No
license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
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For price, delivery, and to place orders: Analog Devices, Inc.,
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Phone: 781-329-4700 • Order online at www.analog.com
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