LM98513CCMTX/NOPB National Semiconductor, LM98513CCMTX/NOPB Datasheet - Page 21

• Manufacturer Part Number: LM98513CCMTX/NOPB
• Description: IC DIGITAL COPIER 10BIT 56TSSOP
• Manufacturer: National Semiconductor

Specifications of LM98513CCMTX/NOPB

• Number Of Bits: 10
• Number Of Channels: 2
• Voltage - Supply, Analog: 3.3V
• Voltage - Supply, Digital: 3.3V
• Package / Case: 56-TFSOP (0.240", 6.10mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power (watts): -
• Lead Free Status / Rohs Status: Compliant

Applications Information
Analog-to-Digital Converter Reference Bypassing
The Analog-to-Digital Converter compares the analog output
voltage from the PGA to a series of reference voltages
developed along an internal reference ladder. The impedance of
this reference ladder is 1000 Ohms. The voltage at the top and
bottom of this ladder are set by internal resistors connected
between the ladder and the positive and negative supplies, as
shown in Figure 19. The top and bottom of the ladder are
available to the user via pins 11,17,32,52. These pins can be
driven by an external reference buffer. If driven, the top
reference voltage (VREFT) should be kept in the range of 2.0 -
2.5V and the bottom reference voltage (VREFB) should be kept
in the range of 0.5 - 1.0V. Whether externally driven, or biased
internally, these pins should be decoupled to ground through
0.1uF capacitors. When biases internally, an additional bypass
capacitor of 10uF should be used between the VREFT and
VREFB pins, as shown in Figure 19. Any device used to drive
the reference pins should be able to source adequate current
into the VREFT pin, and sink adequate current from the VREFB
pin, when the resistor ladder is at its minimum resistivity of 750
ohms.
The voltage difference between the top of the ladder (VREFT)
and the bottom of the ladder (VREFB) may be as low as 1.0V or
as high as 2.0V. However, the performance of the part under
voltage differences greater than or less than the default voltage
reference difference of 1.7V cannot be guaranteed.
Offset Level DAC Reference Bypassing
The offset level DAC reference pins, VREFP and VREFN,
should be capacitively bypassed in the same fashion as the ADC
reference pins VREFT and VREFB (see ”Analog-to-Digital
Converter Reference Bypassing”).
Offset Level DAC Range Adjustment
The offset level DAC has an input range of ±127 LSB (see
Figure 11). This offset level corresponds to 0.5 LSB at the ADC
output per DAC input code step. Therefore, the offset DAC is
limited to providing offset values less than or equal to ±64 LSB at
the ADC output. In some applications, this range of output may
©
2005 National Semiconductor Corporation
Figure 19: Recommended Layout Pattern
21
not be sufficient. It is possible to increase the range of the DAC
by adjusting the DAC reference range. The DAC reference
range may be adjusted by lowering the voltage at the DAC lower
reference pin, VREFN. It is recommended to use a pull-down
resistor from VREFN to AGND to lower the voltage at VREFN. A
resistor value of 200-510Ω will increase the DAC range by a
factor of 2x, allowing offsets of ±127 LSB to be applied at the
ADC output rather than the default maximum and minimum
offsets of ±64 LSB.
Power Supply Considerations
The LM98513 may draw a sufficient amount of current to corrupt
improperly bypassed power supplies. A 10µF to 50µF capacitor
should be placed within 1cm of the analog power (AV+) pins of
the device in parallel with a 0.1µF ceramic chip capacitor placed
as close to the device as layout permits. Leadless chip
capacitors are preferred because they have a low lead
inductance. As is the case with virtually all high-speed
semiconductors, the LM98513 should be assumed to have little
power supply rejection; therefore, a noise-free analog power
source is required.
The analog and digital power supplies of the LM98513 should be
sourced from the same supply voltage, but the supply pins
should be well isolated from one another. Isolating the supplies
prevents digital noise from coupling back into the analog supply
pins. A choke (ferrite bead) is recommended to be placed
between the analog and digital power supply pins as well as a
ceramic chip capacitor placed as close as possible to the analog
supply pin(s) of the device. Additionally, it is not recommended
that the LM98513’s digital supply be used for any other digital
circuitry on the circuit board. All other digital devices should be
powered from a separate digital supply well isolated from both
the analog and digital supplies of the LM98513.
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