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ADS7815

8

INTERMEDIATE LATCHES

The 74HC574s shown in Figure 1 isolate the ADS7815 from

digital signals on a microprocessor, digital signal processor

(DSP), or microcontroller bus. This is necessary because of

the precision needed within the ADS7815. The weight of a

single LSB in the ADS7815 is 76

µV, and the comparator

must be able to resolve differences in voltage to this level.

External digital signals which transition during the conver-

sion can easily couple onto the substrate and produce volt-

ages larger than this.

In place of the 74HC574s, it might be possible to use a FIFO

or similar type of memory device. For the majority of

systems, it will be difficult to go directly from the ADS7815

into a microcontroller or DSP even if the ADS7815 is not

connected to shared bus. The reason for this is that during a

conversion, the ADS7815 outputs are tri-stated. The only

chance to read the outputs are during the acquisition period.

And, this is not recommended if the data will be read just

prior to the converter going into the hold mode.

SIGNAL CONDITIONING

The ADS7815 input essentially consists of a switch and a

capacitor. In the acquisition or sample mode, the switch is

closed and the input signal drives the capacitor directly.

When a conversion is started, the switch is opened capturing

the input signal at that moment. This voltage is held on the

capacitor for the remainder of the conversion.

While this provides for a wide bandwidth sample and hold

function and results in excellent AC performance, this archi-

tecture requires a high bandwidth, precision op amp to drive

the analog input. The op amp and configuration shown in

Figure 1 is highly recommended. The amplifier should be

placed within 1 to 2 inches (25 to 50mm) of the ADS7815,

and the layout guidelines in the OPA628 data sheet should

be strictly followed.