ADC0804LCWMX datasheet(20/48 Pages) TI

20 / 48 page

ADC0801, ADC0802

ADC0803, ADC0804, ADC0805

SNOSBI1B – NOVEMBER 2009 – REVISED FEBRUARY 2013

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Digital Control Inputs

The digital control inputs (CS, RD, and WR) meet standard T2L logic voltage levels. These signals have been

renamed when compared to the standard A/D Start and Output Enable labels. In addition, these inputs are active

low to allow an easy interface to microprocessor control busses. For non-microprocessor based applications, the

CS input (pin 1) can be grounded and the standard A/D Start function is obtained by an active low pulse applied

at the WR input (pin 3) and the Output Enable function is caused by an active low pull at the RD input (pin 2).

Analog Differential Voltage Inputs and Common-Mode Rejection

This A/D has additional applications flexibility due to the analog differential voltage input. The VIN(−) input (pin 7)

can be used to automatically subtract a fixed voltage value from the input reading (tare correction). This is also

useful in 4 mA–20 mA current loop conversion. In addition, common-mode noise can be reduced by use of the

differential input.

The time interval between sampling VIN(+) and VIN(−) is 4-1/2 clock periods. The maximum error voltage due to

this slight time difference between the input voltage samples is given by:

(1)

Where:

∆Ve is the error voltage due to sampling delay

VP is the peak value of the common-mode voltage

fcm is the common-mode frequency

As an example, to keep this error to 1/4 LSB (

∼5 mV) when operating with a 60 Hz common-mode frequency,

fcm, and using a 640 kHz A/D clock, fCLK, would allow a peak value of the common-mode voltage, VP, which is

given by:

(2)

(3)

which gives VP–1.9 V.

The allowed range of analog input voltages usually places more severe restrictions on input common-mode noise

levels.

An analog input voltage with a reduced span and a relatively large zero offset can be handled easily by making

use of the differential input (see Reference Voltage).

Analog Inputs — Input Current

Normal Mode

Due to the internal switching action, displacement currents will flow at the analog inputs. This is due to on-chip

stray capacitance to ground as shown in Figure 49.

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