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AD8551ARZ-REEL

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24
AD8551/AD8552/AD8554 Data Sheet
Rev. F | Page 16 of 24
AD8551/AD8552/AD8554 allows it to quite effectively
minimize offset voltages. The technique also corrects for offset
errors caused by common-mode voltage swings and power
supply variations. This results in superb CMRR and PSRR
figures in excess of 130 dB. Because the autocorrection occurs
continuously, these figures can be maintained across the entire
temperature range of the device, from −40°C to +125°C.
MAXIMIZING PERFORMANCE THROUGH
PROPER LAYOUT
To achieve the maximum performance of the extremely high
input impedance and low offset voltage of the AD8551/
AD8552/AD8554, care is needed in laying out the circuit board.
The PC board surface must remain clean and free of moisture to
avoid leakage currents between adjacent traces. Surface coating
of the circuit board reduces surface moisture and provides a
humidity barrier, reducing parasitic resistance on the board.
The use of guard rings around the amplifier inputs further reduces
leakage currents. Figure 52 shows proper guard ring
configuration, and Figure 53 shows the top view of a surface-
mount layout. The guard ring does not need to be a specific
width, but it should form a continuous loop around both inputs.
By setting the guard ring voltage equal to the voltage at the
noninverting input, parasitic capacitance is minimized as well.
For further reduction of leakage currents, components can be
mounted to the PC board using Teflon standoff insulators.
AD8552
AD8552
AD8552
V
OUT
V
OUT
V
OUT
V
IN
V
IN
V
IN
01101-052
Figure 52. Guard Ring Layout and Connections to Reduce
PC Board Leakage Currents
V+
AD8552
V–
R
2
R
1
R
1
R
2
V
REF
V
REF
V
IN2
GUARD
RING
GUARD
RING
V
IN1
01101-053
Figure 53. Top View of AD8552 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 54 shows a cross-section
of the thermal voltage error sources. If the temperature of the
PC board at one end of the component (T
A1
) is different from
the temperature at the other end (T
A2
), the resulting Seebeck
voltages are not 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 ensures both
Seebeck voltages are equal, thus canceling the thermocouple error.
Maintaining a constant ambient temperature on the circuit board
further reduces this error. The use of a ground plane helps distrib-
ute heat throughout the board and reduces EMI noise pickup.
SOLDER
+
+
+
+
COMPONENT
LEAD
COPPER
TRACE
V
SC1
V
TS1
T
A1
SURFACE-MOUNT
COMPONENT
PC BOARD
T
A2
V
SC2
V
TS2
IF T
A1
≠ T
A2
, THEN
V
TS1
+ V
SC1
≠ V
TS2
+ V
SC2
01101-054
Figure 54. Mismatch in Seebeck Voltages Causes
Thermoelectric Voltage Error
AD8551/
AD8552/
AD8554
A
V
= 1 + (R
F
/R
1
)
NOTES
1. R
S
SHOULD BE PLACED IN CLOSE PROXIMITY AND
ALIGNMENT TO R
1
TO BALANCE SEEBECK VOLTAGES.
R
S
= R
1
R
1
R
F
V
IN
V
OUT
01101-055
Figure 55. Using Dummy Components to Cancel
Thermoelectric Voltage Errors
1/f NOISE CHARACTERISTICS
Another advantage of auto-zero amplifiers is their ability to
cancel flicker noise. Flicker noise, also known as 1/f noise, is
noise inherent in the physics of semiconductor devices, and it
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 AD8551/AD8552/AD8554 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 AD8551/AD8552/AD8554 to have lower noise near
dc than standard low noise amplifiers that are susceptible to 1/f
noise.
Data Sheet AD8551/AD8552/AD8554
Rev. F | Page 17 of 24
INTERMODULATION DISTORTION
The AD8551/AD8552/AD8554 can be used as a conventional
op amp for gain/ bandwidth combinations up to 1.5 MHz. The
auto-zero correction frequency of the device is fixed at 4 kHz.
Although a trace amount of this frequency feeds through to the
output, the amplifier can be used at much higher frequencies.
Figure 56 shows the spectral output of the AD8552 with the
amplifier configured for unity gain and the input grounded.
The 4 kHz auto-zero clock frequency appears at the output with
less than 2 μV of amplitude. Harmonics are also present, but at
reduced levels from the fundamental auto-zero clock frequency.
The amplitude of the clock frequency feedthrough is proportional
to the closed-loop gain of the amplifier. Like other autocorrection
amplifiers, at higher gains there is more clock frequency
feedthrough. Figure 57 shows the spectral output with the
amplifier configured for a gain of 60 dB.
FREQUENCY (kHz)
0
–140
0
101
OUTPUT SIGNAL (dB)
–20
–40
–60
–80
–100
–120
2 3 4
5 6
7
8 9
V
SY
= 5V
A
V
= 0dB
01101-056
Figure 56. Spectral Analysis of AD8552 Output in Unity Gain Configuration
FREQUENCY (kHz)
0
–140
0
101
OUTPUT SIGNAL (dB)
–20
–40
–60
–80
–100
–120
2 3 4 5 6 7 8 9
V
SY
= 5V
A
V
= 60dB
01101-057
Figure 57. Spectral Analysis of AD8551/AD8552/AD8554 Output
with +60 dB Gain
When an input signal is applied, the output contains some
degree of intermodulation distortion (IMD). This is another
characteristic feature of all autocorrection amplifiers. IMD
appears as sum and difference frequencies between the input
signal and the 4 kHz clock frequency (and its harmonics) and is
at a level similar to, or less than, the clock feedthrough at the
output. The IMD is also proportional to the closed-loop gain of
the amplifier. Figure 58 shows the spectral output of an AD8552
configured as a high gain stage (+60 dB) with a 1 mV input signal
applied. The relative levels of all IMD products and harmonic
distortion add up to produce an output error of −60 dB relative
to the input signal. At unity gain, these add up to only −120 dB
relative to the input signal.
IMD < 100µV rms
OUTPUT SIGNAL
1V rms @ 200Hz
FREQUENCY (kHz)
0
0
101
OUTPUT SIGNAL (dB)
–20
–40
–60
–80
–100
–120
2 3 4 5 6 7 8 9
V
SY
= 5V
A
V
= 60dB
01101-058
Figure 58. Spectral Analysis of AD8552 in High Gain with a 1 mV Input Signal
For most low frequency applications, the small amount of auto-
zero clock frequency feedthrough does not affect the precision
of the measurement system. If it is desired, the clock frequency
feedthrough can be reduced through the use of a feedback
capacitor around the amplifier. However, this reduces the
bandwidth of the amplifier. Figure 59 and Figure 60 show a
configuration for reducing the clock feedthrough and the
corresponding spectral analysis at the output. The −3 dB
bandwidth of this configuration is 480 Hz.
100Ω
100kΩ
3.3nF
V
IN
= 1mV rms
@ 200Hz
01101-059
Figure 59. Reducing Autocorrection Clock Noise Using a Feedback Capacitor
FREQUENCY (kHz)
0
0 101
OUTPUT SIGNAL
–20
–40
–60
–80
–100
–120
2 3 4 5 6 7 8 9
V
SY
= 5V
A
V
= 60dB
01101-060
Figure 60. Spectral Analysis Using a Feedback Capacitor
AD8551/AD8552/AD8554 Data Sheet
Rev. F | Page 18 of 24
BROADBAND AND EXTERNAL RESISTOR NOISE
CONSIDERATIONS
The total broadband noise output from any amplifier is primarily
a function of three types of noise: input voltage noise from the
amplifier, input current noise from the amplifier, and Johnson
noise from the external resistors used around the amplifier.
Input voltage noise, or e
n
, is strictly a function of the amplifier
used. The Johnson noise from a resistor is a function of the re-
sistance and the temperature. Input current noise, or i
n
, creates
an equivalent voltage noise proportional to the resistors used
around the amplifier. These noise sources are not correlated
with each other and their combined noise sums in a root-
squared-sum fashion. The full equation is given as
(
)
[
]
2
1
2
2
_
4
S
n
S
n
TOTAL
n
Ri
kTre
e
++
=
(15)
Where:
e
n
= the input voltage noise density of the amplifier.
i
n
= the input current noise of the amplifier.
R
S
= source resistance connected to the noninverting terminal.
k = Boltzmann’s constant (1.38 × 10
−23
J/K).
T = ambient temperature in Kelvin (K = 273.15 + °C).
The input voltage noise density (e
n
) of the AD8551/AD8552/
AD8554 is 42 nV/√Hz, and the input noise, i
n
, is 2 fA/√Hz. The
e
n, TOTAL
is dominated by the input voltage noise, provided the
source resistance is less than 106 kΩ. With source resistance
greater than 106 kΩ, the overall noise of the system is
dominated by the Johnson noise of the resistor itself.
Because the input current noise of the AD8551/AD8552/
AD8554 is very small, it does not become a dominant term
unless R
S
is greater than 4 GΩ, which is an impractical value of
source resistance.
The total noise (e
n, TOTAL
) is expressed in volts per square root
Hertz, and the equivalent rms noise over a certain bandwidth
can be found as
BWee
TOTALn
n
×=
,
(16)
where BW is the bandwidth of interest in Hertz.
OUTPUT OVERDRIVE RECOVERY
The AD8551/AD8552/AD8554 amplifiers have an excellent
overdrive recovery of only 200 μs from either supply rail. This
characteristic is particularly difficult for autocorrection
amplifiers because the nulling amplifier requires a nontrivial
amount of time to error correct the main amplifier back to a
valid output. Figure 29 and Figure 30 show the positive and
negative overdrive recovery times for the AD8551/AD8552/
AD8554.
The output overdrive recovery for an autocorrection amplifier is
defined as the time it takes for the output to correct to its final
voltage from an overload state. It is measured by placing the
amplifier in a high gain configuration with an input signal that
forces the output voltage to the supply rail. The input voltage is
then stepped down to the linear region of the amplifier, usually
to halfway between the supplies. The time from the input signal
stepdown to the output settling to within 100 μV of its final
value is the overdrive recovery time.
INPUT OVERVOLTAGE PROTECTION
Although the AD8551/AD8552/AD8554 are rail-to-rail input
amplifiers, exercise care to ensure that the potential difference
between the inputs does not exceed 5 V. Under normal
operating conditions, the amplifier corrects its output to ensure
the two inputs are at the same voltage. However, if the device is
configured as a comparator, or is under some unusual operating
condition, the input voltages may be forced to different
potentials. This can cause excessive current to flow through
internal diodes in the AD8551/AD8552/AD8554 used to
protect the input stage against overvoltage.
If either input exceeds either supply rail by more than 0.3 V, large
amounts of current begin to flow through the ESD protection
diodes in the amplifier. These diodes connect between the inputs
and each supply rail to protect the input transistors against an
electrostatic discharge event and are normally reverse-biased.
However, if the input voltage exceeds the supply voltage, these
ESD diodes become forward-biased. Without current limiting,
excessive amounts of current can flow through these diodes,
causing permanent damage to the device. If inputs are subjected
to overvoltage, appropriate series resistors should be inserted to
limit the diode current to less than 2 mA maximum.
OUTPUT PHASE REVERSAL
Output phase reversal occurs in some amplifiers when the input
common-mode voltage range is exceeded. As common-mode
voltage moves outside of the common-mode range, the outputs
of these amplifiers suddenly jump in the opposite direction to
the supply rail. This is the result of the differential input pair
shutting down and causing a radical shifting of internal
voltages, resulting in the erratic output behavior.
The AD8551/AD8552/AD8554 amplifiers have been carefully
designed to prevent any output phase reversal, provided both
inputs are maintained within the supply voltages. If there is the
potential of one or both inputs exceeding either supply voltage,
place a resistor in series with the input to limit the current to
less than 2 mA to ensure the output does not reverse its phase.
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AD8551ARZ-REEL

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