AD712 Analog Devices, AD712 Datasheet - Page 15

• Manufacturer Part Number: AD712
• Description: Precision, Low Cost, High Speed, BiFET Dual Op Amp
• Manufacturer: Analog Devices

Specifications of AD712

• -3db Bandwidth: 4MHz
• Slew Rate: 20V/µs
• Vos: 300µV
• Ib: 25pA
• # Opamps Per Pkg: 2
• Input Noise (nv/rthz): 16nV/rtHz
• Vcc-vee: 9V to 36V
• Isy Per Amplifier: 2.8mA
• Packages: DIP,SOIC

Figure 41 and Figure 42 show the settling time characteristics of
the AD712 when used as a DAC output buffer for the AD7545.
NOISE CHARACTERISTICS
The random nature of noise, particularly in the flicker noise
region, makes it difficult to specify in practical terms. At the
same time, designers of precision instrumentation require
certain guaranteed maximum noise levels to realize the full
accuracy of their equipment. All grades of the AD712 are sample
tested on an AQL basis to a limit of 6 μV p-p, 0.1 Hz to 10 Hz.
DRIVING THE ANALOG INPUT OF AN ADC
An op amp driving the analog input of an ADC, such as that
shown in Figure 43, must be capable of maintaining a constant
output voltage under dynamically changing load conditions. In
successive approximation converters, the input current is compared
to a series of switched trial currents. The comparison point is
diode clamped, but can deviate several hundred millivolts resulting
in high frequency modulation of analog-to-digital input current.
The output impedance of a feedback amplifier is made artificially
low by the loop gain. At high frequencies, where the loop gain is
low, the amplifier output impedance can approach its open-loop
value. Most IC amplifiers exhibit a minimum open-loop output
impedance of 25 Ω due to current-limiting resistors.
Figure 42. Negative Settling Characteristics for AD712 with AD7545
Figure 41. Positive Settling Characteristics for AD712 with AD7545
100
100
0%
0%
90
10
90
10
1mV
1mV
5V
5V
500ns
500ns
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ANALOG
A few hundred microamps reflected from the change in converter
loading can introduce errors in instantaneous input voltage. If
the analog-to-digital conversion speed is not excessive and the
bandwidth of the amplifier is sufficient, the amplifier output
returns to the nominal value before the converter makes its
comparison. However, many amplifiers have relatively narrow
bandwidth yielding slow recovery from output transients. The
AD712 is ideally suited to drive high speed ADCs because it
offers both wide bandwidth and high open-loop gain.
INPUT
±10V
Figure 44. ADC Input Unity Gain Buffer Recovery Times, −10 V ADC IN
Figure 45. ADC Input Unity Gain Buffer Recovery Times, −5 V ADC IN
100
100
0%
90
0%
90
10
10
+
–
AD712
Figure 43. AD712 as an ADC Unity-Gain Buffer
+15V
–15V
1/2
1mV
1mV
0.1µF
0.1µF
500mV
500mV
ANALOG COM
ADJUST
OFFSET
ADJUST
100Ω
100Ω
GAIN
R2
R1
–10V ADC IN
–5V ADC IN
PD711 BUFF
PD711 BUFF
12/8
CS
A
R/C
CE
REF IN
REF OUT
BIP OFF
10V
20V
AC
O
IN
IN
AD574A
200ns
200ns
MIDDLE
HIGH
BITS
BITS
LOW
BITS
+15V
–15V
STS
+5V
DC
AD712