Datasheet
ADC78H89
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SNAS201D –APRIL 2003–REVISED MARCH 2013
Figure 24. Typical Application Circuit
ANALOG INPUTS
An equivalent circuit for one of the ADC78H89's input channels is shown in Figure 25. At the start of each
conversion, one of the ADC78H89's seven channels are selected. Diodes D1 and D2 provide ESD protection for
the analog inputs. At no time should an analog input be beyond (AV
DD
+ 300 mV) or (GND - 300 mV), as these
ESD diodes will begin conducting, which could cause erratic operation.
The capacitor C1 in Figure 25 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 25. Equivalent Input Circuit
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 to sample AC signals, a band-pass or low-pass filter
will reduce harmonics and noise, improving dynamic performance.
DIGITAL INPUTS AND OUTPUTS
The ADC78H89's digital inputs (SCLK, CS, and DIN) are limited by and cannot exceed the analog supply voltage
AV
DD
. The digital input pins are not prone to latch-up; SCLK, CS, and DIN may be asserted before DV
DD
without
any risk.
POWER SUPPLY CONSIDERATIONS
The ADC78H89 has two supplies, although they could both have the same potential. There are two major power
supply concerns with this product. They are relative power supply levels, including power-on sequencing, and the
effect of digital supply noise on the analog supply.
Power Management
The ADC78H89 is a dual-supply device. These two supplies share ESD resources, and thus care must be
exercised to ensure that the power supplies are applied in the correct sequence. To avoid turning on the ESD
diodes, the digital supply (DV
DD
) cannot exceed the analog supply (AV
DD
) by more than 300 mV. The
ADC78H89's analog power supply must, therefore, be applied before (or concurrently with) the digital power
supply.
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