AD8510/AD8512/AD8513
Rev. I | Page 13 of 20
GENERAL APPLICATION INFORMATION
INPUT OVERVOLTAGE PROTECTION
The AD8510/AD8512/AD8513 have internal protective
circuitry that allows voltages as high as 0.7 V beyond the
supplies to be applied at the input of either terminal without
causing damage. For higher input voltages, a series resistor is
necessary to limit the input current. The resistor value can be
determined from the formula
mA5≤
−
S
S
IN
R
VV
With a very low offset current of <0.5 nA up to 125°C, higher
resistor values can be used in series with the inputs. A 5 kΩ
resistor protects the inputs from voltages as high as 25 V
beyond the supplies and adds less than 10 µV to the offset.
OUTPUT PHASE REVERSAL
Phase reversal is a change of polarity in the transfer function of
the amplifier. This can occur when the voltage applied at the
input of an amplifier exceeds the maximum common-mode
voltage.
Phase reversal can cause permanent damage to the device and
can result in system lockups. The AD8510/AD8512/AD8513 do
not exhibit phase reversal when input voltages are beyond the
supplies.
TIME (20µs/DIV)
02729-057
VOLTAGE (2V/DIV)
V
IN
V
OUT
V
SY
= ±5V
A
V
= 1
R
L
= 10kΩ
Figure 41. No Phase Reversal
TOTAL HARMONIC DISTORTION (THD) + NOISE
The AD8510/AD8512/AD8513 have low THD and excellent gain
linearity, making these amplifiers great choices for precision
circuits with high closed-loop gain and for audio application
circuits. Figure 42 shows that the AD8510/AD8512/AD8513 have
approximately 0.0005% of total distortion when configured in
positive unity gain (the worst case) and driving a 100 kΩ load.
FREQUENCY (Hz)
DISTORTION (%)
02729-056
0.01
0.001
0.0001
20 100 1k 10k 20k
V
SY
= ±5V
R
L
= 100kΩ
BW = 22kHz
Figure 42. THD + N vs. Frequency
TOTAL NOISE INCLUDING SOURCE RESISTORS
The low input current noise and input bias current of the
AD8510/AD8512/AD8513 make them the ideal amplifiers for
circuits with substantial input source resistance. Input offset
voltage increases by less than 15 nV per 500 Ω of source
resistance at room temperature. The total noise density of the
circuit is
)
SS
nn
nTOTAL
kTRRiee 4
2
2
++=
where:
e
n
is the input voltage noise density of the parts.
i
n
is the input current noise density of the parts.
R
S
is the source resistance at the noninverting terminal.
k is Boltzmann’s constant (1.38 × 10
–23
J/K).
T is the ambient temperature in Kelvin (T = 273 + °C).
For R
S
< 3.9 kΩ, e
n
dominates and e
nTOTAL
≈ e
n
. The current noise
of the AD8510/AD8512/AD8513 is so low that its total density
does not become a significant term unless R
S
is greater than
165 MΩ, an impractical value for most applications.
The total equivalent rms noise over a specific bandwidth is
expressed as
BWee
nTOTALnTOTAL
=
where
BW is the bandwidth in hertz.
Note that the previous analysis is valid for frequencies larger
than 150 Hz and assumes flat noise above 10 kHz. For lower
frequencies, flicker noise (1/f) must be considered.
