AD712JRZ datasheet(14/20 Pages) AD

14 / 20 page

AD712

Rev. G | Page 14 of 20

APPLICATIONS INFORMATION

GUARDING

The low input bias current (15 pA) and low noise characteristics

of the AD712 BiFET op amp make it suitable for electrometer

applications such as photo diode preamplifiers and picoampere

current-to-voltage converters. The use of a guarding technique,

such as that shown in Figure 38, in printed circuit board layout

and construction is critical to minimize leakage currents. The

guard ring is connected to a low impedance potential at the

same level as the inputs. High impedance signal lines should

not be extended for any unnecessary length on the printed

circuit board.

PDIP (N), CERDIP (Q),

AND SOIC (R) PACKAGES.

Figure 38. Board Layout for Guarding Inputs

DIGITAL-TO-ANALOG CONVERTER APPLICATIONS

The AD712 is an excellent output amplifier for CMOS DACs. It

can be used to perform both 2-quadrant and 4-quadrant

operations. The output impedance of a DAC using an inverted

R-2R ladder approaches R for codes containing many 1s, and 3R

for codes containing a single 1. For codes containing all 0s, the

output impedance is infinite.

For example, the output resistance of the AD7545 modulates

between 11 kΩ and 33 kΩ. Therefore, with an 11 kΩ DAC

internal feedback resistance, the noise gain varies from 2 to 4/3.

This changing noise gain modulates the effect of the input offset

voltage of the amplifier, resulting in nonlinear DAC amplifier

performance.

The AD712K with guaranteed 700 μV offset voltage minimizes

this effect to achieve 12-bit performance.

Figure 39 and Figure 40 show the AD712 and AD7545 (12-bit

CMOS DAC) configured for unipolar binary (2-quadrant multi-

plication) or bipolar (4-quadrant multiplication) operation.

Capacitor C1 provides phase compensation to reduce overshoot

and ringing.

+15V

1/2

AD712

VIN

VREF

VDD

RFB

OUT1

AGND

AD7545

DGND

VOUTA

R2A*

DB11 TO DB0

0.1µF

VDD

R1A*

1/2

AD712

VIN

VREF

VDD

RFB

OUT1

AGND

AD7545

DGND

VOUTB

R2B*

DB11 TO DB0

0.1µF

–15V

R1B*

VDD

C1A

33pF

ANALOG

COMMON

*REFER TO

TABLE 3

GAIN

ADJUST

C1B

33pF

ANALOG

COMMON

GAIN

ADJUST

*REFER TO

TABLE 3

Figure 39. Unipolar Binary Operation

R1 and R2 calibrate the zero offset and gain error of the DAC.

Specific values for these resistors depend upon the grade of

AD7545 and are listed in Table 3.

Table 3. Recommended Trim Resistor Values vs. Grades of

the AD7545 for VDD = 5 V

Trim

Resistor

JN/AQ

KN/BQ

GLN

500 Ω

200 Ω

100 Ω

20 Ω

150 Ω

68 Ω

33 Ω

6.8 Ω

+15V

1/2

AD712

VIN

VREF

RFB

OUT1

AGND

AD7545

DGND

R2*

DATA INPUT

0.1µF

VDD

R1*

1/2

AD712

VOUT

0.1µF

–15V

DB11 TO DB0

20kΩ 1%

10kΩ 1%

33pF

ANALOG

COMMON

VDD

GAIN

ADJUST

*FOR VALUES OF

R1 AND R2 SEE TABLE 3

Figure 40. Bipolar Operation

AD712JRZ датащи(PDF) 14 Page - Analog Devices

AD712JRZ датащи(HTML) 14 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

датащи скачать

ссылки URL