AD711CQ AD [Analog Devices], AD711CQ Datasheet - Page 9

• Manufacturer Part Number: AD711CQ
• Description: Precision, Low Cost, High Speed, BiFET Op Amp
• Manufacturer: AD [Analog Devices]

REV. A
GUARDING
The low input bias current (15 pA) and low noise characteristics
of the AD711 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 30, 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.
Figure 28a. Settling Characteristics 0 to +10 V Step
Upper Trace: Output of AD711 Under Test (5 V/Div)
Lower Trace: Amplified Error Voltage (0.01%/Div)
Figure 30. Board Layout for Guarding Inputs
Figure 29. Settling Time Test Circuit
–9–
D/A CONVERTER APPLICATIONS
The AD711 is an excellent output amplifier for CMOS DACs.
It can be used to perform both 2 quadrant and 4 quadrant op-
eration. The output impedance of a DAC using an inverted
R-2R ladder approaches R for codes containing many 1s, 3R for
codes containing a single 1, and for codes containing all zero,
the output impedance is infinite.
For example, the output resistance of the AD7545 will modu-
late between 11 k and 33 k . Therefore, with the DAC’s in-
ternal feedback resistance of 11 k , the noise gain will vary 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 AD711K with guaranteed 500 V offset voltage minimizes
this effect to achieve 12-bit performance.
Figures 31 and 32 show the AD711 and AD7545 (12-bit
CMOS DAC) configured for unipolar binary (2-quadrant multi-
plication) or bipolar (4-quadrant multiplication) operation. Ca-
pacitor C1 provides phase compensation to reduce overshoot
and ringing.
Figure 28b. Settling Characteristics 0 to –10 V Step
Upper Trace: Output of AD711 Under Test (5 V/Div)
Lower Trace: Amplified Error Voltage (0.01%/Div)
AD711