ADC12062EVAL National Semiconductor, ADC12062EVAL Datasheet - Page 17

• Manufacturer Part Number: ADC12062EVAL
• Description: BOARD EVALUATION FOR ADC12062
• Manufacturer: National Semiconductor

Specifications of ADC12062EVAL

• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Other names: *ADC12062EVAL

Applications Information
Figure 12 gives an example of a suitable layout, including
power supply routing, ground plane separation, and bypass
capacitor placement. All analog circuitry (input amplifiers,
filters, reference components, etc.) should be placed on the
analog ground plane. All digital circuitry and I/O lines (ex-
cluding the S/H input) should use the digital2 ground plane
as ground. The digital1 ground plane should only be used for
the S/H signal generation.
7.0 DYNAMIC PERFORMANCE
The ADC12062 is AC tested and its dynamic performance is
guaranteed. In order to meet these specifications, the clock
source driving the S/H input must be free of jitter. For the
best AC performance, a crystal oscillator is recommended.
For operation at or near the ADC12062’s 1 MHz maximum
sampling rate, a 1 MHz squarewave will provide a good
signal for the S/H input. As long as the duty cycle is near
50%, the waveform will be low for about 500 ns, which is
within the 550 ns limit. When operating the ADC12062 at a
sample rate of 910 kHz or below, the pulse width of the S/H
signal must be smaller than half the sample period.
FIGURE 12. PC Board Layout
(Continued)
01149022
17
Figure 13 is an example of a low jitter S/H pulse generator
that can be used with the ADC12062 and allow operation at
sampling rates from DC to 1 MHz. A standard 4-pin DIP
crystal oscillator provides a stable 1 MHz squarewave. Since
most DIP oscillators have TTL outputs, a 4.7k pullup resistor
is used to raise the output high voltage to CMOS input levels.
The output is fed to the trigger input (falling edge) of an
MM74HC4538 one-shot. The 1k resistor and 12 pF capacitor
set the pulse length to approximately 100 ns. The S/H pulse
stream for the converter appears on the Q output of the
HC4538. This is the S/H clock generator used on the
ADC12062EVAL evaluation board. For lower power, a
CMOS inverter-based crystal oscillator can be used in place
of the DIP crystal oscillator. See Application Note AN-340 in
the National Semiconductor CMOS Logic Databook for more
information on CMOS crystal oscillators.
8.0 COMMON APPLICATION PITFALLS
Driving inputs (analog or digital) outside power supply
rails. The Absolute Maximum Ratings state that all inputs
must be between GND − 300 mV and V
rule is most often broken when the power supply to the
converter is turned off, but other devices connected to it (op
amps, microprocessors) still have power. Note that if there is
no power to the converter, DGND = AGND = DV
DGND.
Driving a high capacitance digital data bus. The more
capacitance the data bus has to charge for each conversion,
the more instantaneous digital current required from DV
and DGND. These large current spikes can couple back to
the analog section, decreasing the SNR of the converter.
While adequate supply bypassing and separate analog and
digital ground planes will reduce this problem, buffering the
digital data outputs (with a pair of MM74HC541s, for ex-
ample) may be necessary if the converter must drive a
heavily loaded databus.
= 0V, so all inputs should be within
FIGURE 13. Crystal Clock Source
±
300 mV of AGND and
CC
+ 300 mV. This
www.national.com
CC
= AV
01149023
CC
CC