ADC78H89CIMT National Semiconductor, ADC78H89CIMT Datasheet - Page 14

• Manufacturer Part Number: ADC78H89CIMT
• Description: IC, 12BIT ADC 500KSPS, POWERWISE
• Manufacturer: National Semiconductor

Specifications of ADC78H89CIMT

• Resolution (bits): 12bit
• Input Channel Type: Single Ended
• Data Interface: Serial, SPI
• Supply Voltage Range - Analogue: 2.7V To 5.25V
• Supply Voltage Range - Digital: -0.3V To 5.25V, 2.7V To 5.25V
• Sampling Rate: 500kSPS
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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Applications Information
3.0 ADC78H89 TRANSFER FUNCTION
The output format of the ADC89H89 is straight binary. Code
transitions occur midway between successive integer LSB
values. The LSB width for the ADC78H89 is AV
The ideal transfer characteristic is shown in Figure 4.
5.0 ANALOG INPUTS
An equivalent circuit for one of the ADC78H89’s input chan-
nels is shown in Figure 6. At the start of each conversion,
one of the ADC78H89’s seven channels are selected. Di-
odes D1 and D2 provide ESD protection for the analog
inputs. At no time should an analog input be beyond (AV
300 mV) or (GND - 300 mV), as these ESD diodes will begin
conducting, which could cause erratic operation.
The capacitor C1 in Figure 6 typically has a value of 3 pF,
and is mainly the package pin capacitance. Resistor R1 is
the on resistance of the multiplexer and track / hold switch,
and is typically 500 ohms. Capacitor C2 is the ADC78H89
sampling capacitor, and is typically 30 pF. The ADC78H89
will deliver best performance when driven by a low-
impedance source to eliminate distortion caused by the
charging of the sampling capacitor.
FIGURE 4. Ideal Transfer Characteristic
FIGURE 5. Typical Application Circuit
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4.0 TYPICAL APPLICATION CIRCUIT
A typical application of the ADC78H89 is shown in Figure 5.
The split analog and digital supplies are both provided in this
example by the National LP2950 low-dropout voltage regu-
lator, available in a variety of fixed and adjustable output
voltages. The analog supply is bypassed with a capacitor
network located close to the ADC78H89. The digital supply
is separated from the analog supply by an isolation resistor
and conditioned with additional bypass capacitors. The
ADC78H89 uses the analog supply (AV
voltage, so it is very important that AV
possible. Because of the ADC78H89’s low power require-
ments, it is also possible to use a precision reference as a
power supply to maximize performance. The four-wire inter-
face is also shown connected to a microprocessor or DSP.
In applications where dynamic performance is critical, the
ADC78H89 might need to be driven with a low output-
impedance amplifier. In addition, when using the ADC78H89
FIGURE 6. Equivalent Input Circuit
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be kept as clean as
) as its reference
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