AD7940

Rev. 0 | Page 12 of 20

ADC TRANSFER FUNCTION

The output coding of the AD7940 is straight binary. The de-

signed code transitions occur at successive integer LSB values,

characteristic for the AD7940 is shown in Figure 14.

000...000

111...111

1 LSB

ANALOG INPUT

0V

000...001

000...010

111...110

111...000

011...111

Figure 14. AD7940 Transfer Characteristic

TYPICAL CONNECTION DIAGRAM

Figure 15 shows a typical connection diagram for the AD7940.

The conversion result is output in a 16-bit word. This 16-bit

data stream consists of two leading zeros, followed by the 14 bits

of conversion data, MSB first. For applications where power

consumption is a concern, the power-down mode should be

used between conversions or bursts of several conversions to

improve power performance (see the Modes of Operation

section).

In fact, because the supply current required by the AD7940 is so

low, a precision reference can be used as the supply source to

the AD7940. For example, a REF19x voltage reference (REF195

for 5 V or REF193 for 3 V) or an AD780 can be used to supply

the required voltage to the ADC (see Figure 15). This configura-

tion is especially useful if the power supply available is quite

noisy, or if the system supply voltages are at some value other

than the required operating voltage of the AD7940, e.g., 15 V.

The REF19x or AD780 will output a steady voltage to the

AD7940. Recommended decoupling capacitors are a 100 nF low

ESR ceramic (Farnell 335-1816) and a 10 µF low ESR tantalum

(Farnell 197-130).

AD7940

INPUT

SCLK

SDATA

SERIAL

INTERFACE

µC/µP

CS

GND

10

µF

TANT

0.1

µF

3V

10

µF

0.1

µF

5V

SUPPLY

REF193

Figure 15. Typical Connection Diagram

Digital Inputs

The digital inputs applied to the AD7940 are not limited by the

maximum ratings that limit the analog inputs. Instead, the digi-

tal inputs applied can go to 7 V and are not restricted by the

be used on the digital inputs. However, it is important to note

that the data output on SDATA will still have 3 V logic levels

Another advantage of SCLK and CS not being restricted by the

there is no risk of latch-up as there would be on the analog