AD8571ARM Просмотр технического описания (PDF) - Analog Devices
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AD8571ARM Datasheet PDF : 19 Pages
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AD8571/AD8572/AD8574
VOUT
VIN
AD8572
VIN
VOUT
AD8572
VIN
VOUT
AD8572
Figure 46. Guard Ring Layout and Connections to Reduce
PC Board Leakage Currents
R1
VIN1
V+
R2
AD8572
R2
R1
VIN2
GUARD
RING
VREF
GUARD
RING
VREF
V؊
Figure 47. Top View of AD8572 SOIC Layout with
Guard Rings
Other potential sources of offset error are thermoelectric voltages
on the circuit board. This voltage, also called Seebeck voltage,
occurs at the junction of two dissimilar metals and is proportional
to the temperature of the junction. The most common metallic
junctions on a circuit board are solder-to-board trace and solder-
to-component lead. Figure 48 shows a cross-section diagram view
of the thermal voltage error sources. If the temperature of the PC
board at one end of the component (TA1) is different from the
temperature at the other end (TA2), the Seebeck voltages will not
be equal, resulting in a thermal voltage error.
This thermocouple error can be reduced by using dummy com-
ponents to match the thermoelectric error source. Placing the
dummy component as close as possible to its partner will ensure
both Seebeck voltages are equal, thus canceling the thermo-
couple error. Maintaining a constant ambient temperature on
the circuit board will further reduce this error. The use of a
ground plane will help distribute heat throughout the board and
will also reduce EMI noise pickup.
COMPONENT
LEAD
VSC1
VTS1
SURFACE MOUNT
COMPONENT
PC BOARD
SOLDER
+ VSC2
؊ VTS2
+
؊
TA1
COPPER
TRACE
TA2
IF TA1 = TA2, THEN
VTS1 + VSC1 = VTS2 + VSC2
Figure 48. Mismatch in Seebeck Voltages Causes a
Thermoelectric Voltage Error
RF
R1
VIN
RS = R1
AD857x
VOUT
AV = 1 + (RF /R1)
NOTE: RS SHOULD BE PLACED IN CLOSE PROXIMITY AND
ALIGNMENT TO R1 TO BALANCE SEEBECK VOLTAGES
Figure 49. Using Dummy Components to Cancel
Thermoelectric Voltage Errors
1/f Noise Characteristics
Another advantage of autozero amplifiers is their ability to cancel
flicker noise. Flicker noise, also known as 1/f noise, is noise inher-
ent in the physics of semiconductor devices and increases 3 dB
for every octave decrease in frequency. The 1/f corner frequency
of an amplifier is the frequency at which the flicker noise is equal
to the broadband noise of the amplifier. At lower frequencies,
flicker noise dominates, causing higher degrees of error for sub-
Hertz frequencies or dc precision applications.
Because the AD857x amplifiers are self-correcting op amps,
they do not have increasing flicker noise at lower frequencies.
In essence, low frequency noise is treated as a slowly varying
offset error and is greatly reduced as a result of autocorrection.
The correction becomes more effective as the noise frequency
approaches dc, offsetting the tendency of the noise to increase
exponentially as frequency decreases. This allows the AD857x
to have lower noise near dc than standard low-noise amplifiers
that are susceptible to 1/f noise.
Random Autozero Correction Eliminates Intermodulation
Distortion
The AD857x can be used as a conventional op amp for gains up
to 1 MHz. The autozero correction frequency of the device
continuously varies, based on a pseudo-random generator with a
uniform distribution from 2 kHz to 4 kHz. The randomization
of the autocorrection clock creates a continuous randomization
of intermodulation distortion (IMD) products, which show up
as simple broadband noise at the output of the amplifier. This
noise naturally combines with the amplifier’s voltage noise in a
root-squared-sum fashion, resulting in an output free of IMD.
Figure 50a shows the spectral output of an AD8572 with the
amplifier configured for unity gain and the input grounded. Figure
50b shows the spectral output with the amplifier configured for a
gain of 60 dB.
–12–
REV. 0
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