ADC0806 National Semiconductor, ADC0806 Datasheet - Page 18

• Manufacturer Part Number: ADC0806
• Description: 500 ns A/D Converter with S/H Function and Input Multiplexer
• Manufacturer: National Semiconductor

www.national.com
Applications Information
Figure 5 gives an example of a suitable layout. All analog
circuitry (input amplifiers, filters, reference components, etc.)
should be placed together away from any digital compo-
nents.
6.0 DYNAMIC PERFORMANCE
The ADC08060 is ac tested and its dynamic performance is
guaranteed. To meet the published specifications, the clock
source driving the CLK input must exhibit less than 10 ps
(rms) of jitter. For best ac performance, isolating the ADC
clock from any digital circuitry should be done with adequate
buffers, as with a clock tree. See Figure 6.
It is good practice to keep the ADC clock line as short as
possible and to keep it well away from any other signals.
Other signals can introduce jitter into the clock signal. The
clock signal can also introduce noise into the analog path.
7.0 COMMON APPLICATION PITFALLS
Driving the inputs (analog or digital) beyond the power
supply rails. For proper operation, all inputs should not go
more than 300 mV below the ground pins or 300 mV above
the supply pins. Exceeding these limits on even a transient
FIGURE 6. Isolating the ADC Clock from Digital
Circuitry
(Continued)
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basis may cause faulty or erratic operation. It is not uncom-
mon for high speed digital circuits (e.g., 74F and 74AC
devices) to exhibit undershoot that goes more than a volt
below ground. A 51Ω resistor in series with the offending
digital input will usually eliminate the problem.
Care should be taken not to overdrive the inputs of the
ADC08060. Such practice may lead to conversion inaccura-
cies and even to device damage.
Attempting to drive a high capacitance digital data bus.
The more capacitance the output drivers must charge for
each conversion, the more instantaneous digital current is
required from DR V
current spikes can couple into the analog section, degrading
dynamic performance. Buffering the digital data outputs (with
a 74F541, for example) may be necessary if the data bus
capacitance exceeds 10 pF. Dynamic performance can also
be improved by adding 47Ω to 56Ω series resistors at each
digital output, reducing the energy coupled back into the
converter input pins.
Using an inadequate amplifier to drive the analog input.
As explained in Section 2.0, the capacitance seen at the
input alternates between 3 pF and 4 pF with the clock. This
dynamic capacitance is more difficult to drive than is a fixed
capacitance, and should be considered when choosing a
driving device. The LMH6702 has been found to be a good
device for driving the ADC08060.
Driving the V
not source or sink the current required by the ladder.
As mentioned in Section 1.0, care should be taken to see
that any driving devices can source sufficient current into the
V
pins are not driven with devices than can handle the required
current, these reference pins will not be stable, resulting in a
reduction of dynamic performance.
Using a clock source with excessive jitter, using an
excessively long clock signal trace, or having other
signals coupled to the clock signal trace. This will cause
the sampling interval to vary, causing excessive output noise
and a reduction in SNR performance. The use of simple
gates with RC timing is generally inadequate as a clock
source.
RT
pin and sink sufficient current from the V
RT
pin or the V
D
and DR GND. These large charging
RB
pin with devices that can
RB
pin. If these