PCM78 Burr-Brown Corporation, PCM78 Datasheet - Page 14

• Manufacturer Part Number: PCM78
• Description: 16-Bit Audio ANALOG-TO-DIGITAL CONVERTER
• Manufacturer: Burr-Brown Corporation

FIGURE 13. Recommended Power Supply Decoupling.
Reference Decoupling and Speed Up
In order to assure the lowest noise operation of the PCM78,
the reference may be bypassed by three different capacitors.
Pin 25 is a decoupling point for the reference to –V
If Short Cycle is not held low until the next convert
command is issued, the Status line will go high in
synchronization with Short Cycle. This is because the
operation of the Status line becomes invalid after Short
Cycle is asserted. An example of the Short Cycle operation
is shown in Figure 12.
In those systems where a user may not be using a continuous
external clock, it is necessary to assure that a falling edge
of external clock occurs after short cycle goes low. This is
because conversion actually stops on the first falling edge
of external clock after Short Cycle goes low.
ANALOG CIRCUIT CONSIDERATIONS
Layout Precautions
Analog and Digital Common are connected internally in the
PCM78, and should be connected together as close to the
unit as possible, preferably to a large ground plane under the
ADC. Low impedance analog and digital common returns
are essential for low noise performance. Coupling between
analog inputs and digital lines should be minimized by
careful layout. The input pin (pin 1) and the MSB adjust pin
(pin 3) are both extremely sensitive to noise; digital lines
should be kept away from these pins to avoid coupling
digital noise into the sensitive analog circuitry.
Contact factory for a recommended PCB layout for the
PCM78.
Power Supply Decoupling
The power supplies should be bypassed with tantalum or
electrolytic capacitors as shown in Figure 13 to obtain noise
free operation. These capacitors should be located as close
to the ADC as possible. Bypass the 1 F electrolytic
capacitors with 0.01 F ceramic or polystyrene capacitors
for improved high frequency performance.
®
1µF
PCM78
+
+V
23
.01µF
PCM78
CC
+
+
1µF
–V
2
.01µF
CC
4
+
+
1µF
+V
DD
.01µF
CC
. The
14
decoupling capacitor should range from 0.1 F to 4.7 F;
larger values can cause reference settling problems which
may manifest themselves as missing codes. This capacitor
should be as close to the PCM78 as possible, to minimize
the potential for coupling noise into the device; with a good
board layout it may be best to leave this capacitor out of the
circuit altogether, as the extra lead length may only cause
more noise in the reference.
Pin 27 is a decoupling point to ground, as well as the output
of the 2V reference. This point should not be used to supply
reference voltage to external circuitry unless it is buffered.
A 2.2 F capacitor is recommended, and the capacitor used
here should not exceed 4.7 F.
Pin 28, the Speed Up pin, allows a capacitor to be connected
to ground to facilitate reference settling. This does not speed
up the conversion time, but it does reduce odd order
harmonic distortion. As with the decoupling capacitor on
pin 25, this may also contribute to noise; if harmonic content
is most important in an application, this capacitor (0.1 F -
10 F) should be connected. In all other cases, it is best to
leave the capacitor out of the circuit.
Input Scaling
The analog input should be scaled as close to the maximum
input signal range as possible in order to utilize the
maximum signal resolution of the A/ D converter. The DAC
inside the PCM78 has a 2mA range, and the nominal 3V
input is scaled by a 1.5k resistor. In order to scale to other
ranges, see Table I for recommended scaling resistor values,
connected as shown in Figure 14.
TABLE I. PCM78 Input Scaling Resistor Values.
FIGURE 14. PCM78 Input Scaling Circuit.
INPUT IMPEDANCE
The input signal to the PCM78 should come from a low
impedance source, such as the output of an op amp, to avoid
any errors due to the dynamic input impedance that a
successive-approximation converter presents to the outside
world because of the changing currents in this circuit during
conversion as the converter steps through its approxima-
tions.
If the driving circuit output impedance is not low, a buffer
amplifier should be added between the input signal and the
direct input to the PCM78 as shown in Figure 15.
NOTE: R values shown assume use of 1k trim pot to adjust for scale
accuracy.
*Use to trim for exact scaling. Use
trim pot with temperature coefficient
of 100ppm/°C or better.
INPUT RANGE
10V
5V
*1k
R
1
8.2k
3.3k
PCM78
R