Instruction Manual

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Applications Information (Continued)

3.3 PD

The PD pin, when high, holds the ADC12L080 in a power-

down mode to conserve power when the converter is not

being used. The power consumption in this state is 50 mW

and is not affected by the clock frequency, or by whether

there is a clock signal present. The output data pins are

undefined and the data in the pipeline is corrupted while in

the power down mode.

The Power Down Mode Exit Cycle time is determined by the

value of the capacitors on pins 30, 31 and 32. These capaci-

tors loose their charge in the Power Down mode and must

be recharged by on-chip circuitry before conversions can be

accurate. See Section 2.1

4.0 OUTPUTS

The ADC12L080 has 12 TTL/CMOS compatible Data Output

pins. The output data is present at these outputs while the

PD pin is low. While the t

time provides information about

output timing, a simple way to capture a valid output is to

latch the data on the rising edge of the conversion clock (pin

10).

Be very careful when driving a high capacitance bus. The

more capacitance the output drivers must charge for each

conversion, the more instantaneous digital current flows

through V

and DR GND. These large charging current

spikes can cause on-chip noise and couple into the analog

circuitry, degrading dynamic performance. Adequate by-

passing, limiting output capacitance and careful attention to

the ground plane will reduce this problem. Additionally, bus

capacitance beyond the specified 15 pF/pin will cause t

increase, making it difficult to properly latch the ADC output

data. The result could be an apparent reduction in dynamic

performance.

To minimize noise due to output switching, minimize the load

currents at the digital outputs. This can be done by connect-

ing buffers between the ADC outputs and any other circuitry

(74ACQ541, for example). Only one driven input should be

connected to each output pin. Additionally, inserting series

100Ω resistors at the digital outputs, close to the ADC pins,

will isolate the outputs from trace and other circuit capaci-

tances and limit the output currents, which could otherwise

result in performance degradation. See Figure 4.

While the ADC12L080 will operate with V

voltages down

to 1.8V, t

increases with reduced V

. Be careful of

external timing when using reduced V

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FIGURE 4. Simple Application Circuit with Single-Ended to Differential Buffer

ADC12L080