Datasheet AD712 (Analog Devices) - 14

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AD712 Data Sheet APPLICATIONS INFORMATION GUARDING
Figure 39 and Figure 40 show the AD712 and AD7545 (12-bit The low input bias current (15 pA) and low noise characteristics CMOS DAC) configured for unipolar binary (2-quadrant multi- of the AD712 BiFET op amp make it suitable for electrometer plication) or bipolar (4-quadrant multiplication) operation. applications such as photo diode preamplifiers and picoampere Capacitor C1 provides phase compensation to reduce overshoot current-to-voltage converters. The use of a guarding technique, and ringing. such as that shown in Figure 38, in printed circuit board (PCB)
VDD R2A*
layout and construction is critical to minimize leakage currents.
+15V C1A 0.1µF
The guard ring is connected to a low impedance potential at the
33pF GAIN V
same level as the inputs. High impedance signal lines should
DD RFB ADJUST – OUT1
not be extended for any unnecessary length on the PCB.
1/2 VIN V AD7545 REF AD + 712 VOUTA R1A* AGND PDIP (N), CERDIP (Q), DGND AND SOIC (R) PACKAGES. ANALOG *REFER TO 4 COMMON 5 TABLE 3 DB11 TO DB0 6 3 V R2B* DD 2 7
8
C1B
-03
33pF 8 1
823 00
GAIN VDD RFB ADJUST –
Figure 38. Board Layout for Guarding Inputs
OUT1 1/2 VIN V AD7545 REF AD712 VOUTB R1B* + DAC CONVERTER APPLICATIONS AGND DGND 0.1µF
The AD712 is an excellent output amplifier for CMOS DACs. It can
ANALOG *REFER TO COMMON
be used to perform both 2-quadrant and 4-quadrant operations.
TABLE 3
039
–15V
23-
DB11 TO DB0
008 The output impedance of a DAC using an inverted R-2R ladder Figure 39. Unipolar Binary Operation approaches R for codes containing many 1s, and 3R for codes containing a single 1. For codes containing all 0s, the output R1 and R2 calibrate the zero offset and gain error of the DAC. impedance is infinite. Specific values for these resistors depend upon the grade of AD7545 and are listed in Table 4. For example, the output resistance of the AD7545 modulates between 11 kΩ and 33 kΩ. Therefore, with an 11 kΩ DAC
Table 4. Recommended Trim Resistor Values vs. Grades of
internal feedback resistance, the noise gain varies from 2 to 4/3.
the AD7545 fo r VDD = 5 V
This changing noise gain modulates the effect of the input offset
Trim
voltage of the amplifier, resulting in nonlinear DAC amplifier
Resistor JN/AQ KN/BQ LN GLN
performance. R1 500 Ω 200 Ω 100 Ω 20 Ω The AD712K with guaranteed 700 μV offset voltage minimizes R2 150 Ω 68 Ω 33 Ω 6.8 Ω this effect to achieve 12-bit performance.
V R2* DD R4 20kΩ 1% C1 +15V 0.1µF 33pF R5 GAIN V R 20kΩ 1% DD FB ADJUST – OUT1 1/2 – V AD7545 IN VREF A + D712 R1* 1/2 AGND R3 AD712 VOUT 10kΩ 1% DGND + DB11 TO DB0 0.1µF 12 DATA INPUT –15V *FOR VALUES OF ANALOG
-040
R1 AND R2 SEE TABLE 3
23
COMMON
008 Figure 40. Bipolar Operation Rev. I | Page 14 of 20 Document Outline FEATURES CONNECTION DIAGRAM GENERAL DESCRIPTION PRODUCT HIGHLIGHTS TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE ESD CAUTION TYPICAL PERFORMANCE CHARACTERISTICS SETTLING TIME OPTIMIZING SETTLING TIME OP AMP SETTLING TIME—A MATHEMATICAL MODEL APPLICATIONS INFORMATION GUARDING DAC CONVERTER APPLICATIONS NOISE CHARACTERISTICS DRIVING THE ANALOG INPUT OF AN ADC DRIVING A LARGE CAPACITIVE LOAD FILTERS ACTIVE FILTER APPLICATIONS SECOND-ORDER LOW-PASS FILTER 9-POLE CHEBYCHEV FILTER OUTLINE DIMENSIONS ORDERING GUIDE