AN2438 Freescale Semiconductor / Motorola, AN2438 Datasheet - Page 15

• Manufacturer Part Number: AN2438
• Description: ADC Definitions and Specifications
• Manufacturer: Freescale Semiconductor / Motorola

Acquisition Error (E
MOTOROLA
AQ
The best way to eliminate this error is to reduce the Analog DC Source
Resistance and any form of leakage within the customer’s control (PC Board
leakage). An active circuit (op-amp) that buffers the Input Voltage (V
reduce Analog DC Source Resistance, but the problem may just shift to the
input of the active circuit.
Acquisition Error is the error due to the inability of the acquisition circuits to fully
charge the input of the ADC in the time allotted. The Acquisition Error will
depend on the type of acquisition circuit the ADC uses:
The Acquisition Error is related to the difference between the required
Acquisition time and the allowed acquisition time.
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Fixed Acquisition Time Sample and Hold Acquisition — This type of
circuit opens the input of the ADC to the Analog Input for a fixed period
of time, usually some number of conversion clock cycles. In these cases,
the Analog AC Source Impedance is critical since it will have to be less
than a specified amount in order for the input to charge in the given time.
Many ADC’s from Motorola use this architecture.
Fixed Acquisition Time Sample and Hold with Buffered Input
Acquisition — This type of circuit is similar to the Fixed Acquisition
Time architecture only for the first few cycles of the acquisition period,
an op-amp buffers the input so that the ADC charges much faster.
During the last few cycles of the acquisition period, the buffer is disabled
and the Analog Input Source charges the ADC input the rest of the way.
This architecture is nominally faster than the other for a given source
impedance, but if the offset in the op-amp is high then the total charge
time (from the input-offset voltage to the correct input voltage) may still
be dependant on the AC source impedance. Most remaining Motorola
ADC’s use this architecture.
Variable Acquisition Time Sample and Hold Acquisition — This type
of circuit uses a variable period — either software controlled or
controlled through a programmable timer — for the acquisition phase. In
this manner, high Analog Source Impedance can be compatible with the
ADC, at the expense of acquisition time. Since the time is variable, then
in cases where there is low Analog Source Impedance the acquisition
time can be as fast as possible. Many competitors use this form of
acquisition.
Continuous Acquisition — This type of circuit continually applies the
input voltage to the ADC’s comparator input. Few 10b ADC architectures
are compatible with this method (full-RDAC SAR architectures can use
this type of acquisition but are usually limited to about 7b of accuracy).
ADC Definitions and Specifications
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Acquisition Error (EAQ)
IN
AN2438/D
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