LT1028_01 Datasheet, PDF (10/20 Pages) Linear Technology – Ultralow Noise Precision High Speed Op Amps

LT1028/LT1128
APPLICATI
U
S I FOR ATIO – OISE
Voltage Noise vs Current Noise
The LT1028/LT1128’s less than 1nV/√Hz voltage noise is
three times better than the lowest voltage noise heretofore
available (on the LT1007/1037). A necessary condition for
such low voltage noise is operating the input transistors at
nearly 1mA of collector currents, because voltage noise is
inversely proportional to the square root of the collector
current. Current noise, however, is directly proportional to
the square root of the collector current. Consequently, the
LT1028/LT1128’s current noise is significantly higher
than on most monolithic op amps.
Therefore, to realize truly low noise performance it is
important to understand the interaction between voltage
noise (e_n), current noise (I_n) and resistor noise (r_n).
Total Noise vs Source Resistance
The total input referred noise of an op amp is given by
e_t = [e_n² + r_n² + (I_nR_eq)²]^1/2
where R_eq is the total equivalent source resistance at the
two inputs, and
r_n = √4kTR_eq = 0.13√R_eq in nV/√Hz at 25°C
As a numerical example, consider the total noise at 1kHz
of the gain 1000 amplifier shown below.
100Ω
100k
100Ω
–
LT1028
LT1128
+
1028/1128 AI01
R_eq = 100Ω + 100Ω || 100k ≈ 200Ω
r_n = 0.13√200 = 1.84nV√Hz
e_n = 0.85nV√Hz
I_n = 1.0pA/√Hz
e_t = [0.85² + 1.84² + (1.0 × 0.2) ²]^1/2 = 2.04nV/√Hz
Output noise = 1000 e_t = 2.04µV/√Hz
At very low source resistance (R_eq < 40Ω) voltage noise
dominates. As R_eq is increased resistor noise becomes the
largest term, as in the example above, and the LT1028/
LT1128’s voltage noise becomes negligible. As R_eq is
further increased, current noise becomes important. At
1kHz, when R_eq is in excess of 20k, the current noise
component is larger than the resistor noise. The total noise
versus matched source resistance plot illustrates the
above calculations.
The plot also shows that current noise is more dominant
at low frequencies, such as 10Hz. This is because resistor
noise is flat with frequency, while the 1/f corner of current
noise is typically at 250Hz. At 10Hz when R_eq > 1k, the
current noise term will exceed the resistor noise.
When the source resistance is unmatched, the total noise
versus unmatched source resistance plot should be con-
sulted. Note that total noise is lower at source resistances
below 1k because the resistor noise contribution is less.
When R_S > 1k total noise is not improved, however. This
is because bias current cancellation is used to reduce
input bias current. The cancellation circuitry injects two
correlated current noise components into the two inputs.
With matched source resistors the injected current noise
creates a common-mode voltage noise and gets rejected
by the amplifier. With source resistance in one input only,
the cancellation noise is added to the amplifier’s inherent
noise.
In summary, the LT1028/LT1128 are the optimum ampli-
fiers for noise performance, provided that the source
resistance is kept low. The following table depicts which
op amp manufactured by Linear Technology should be
used to minimize noise, as the source resistance is in-
creased beyond the LT1028/LT1128’s level of usefulness.
Best Op Amp for Lowest Total Noise vs Source Resistance
SOURCE RESIS-
TANCE(Ω) (Note 1)
BEST OP AMP
AT LOW FREQ(10Hz) WIDEBAND(1kHz)
0 to 400
400 to 4k
4k to 40k
40k to 500k
500k to 5M
>5M
LT1028/LT1128
LT1007/1037
LT1001
LT1012
LT1012 or LT1055
LT1055
LT1028/LT1128
LT1028/LT1128
LT1007/1037
LT1001
LT1012
LT1055
Note 1: Source resistance is defined as matched or unmatched, e.g.,
R_S = 1k means: 1k at each input, or 1k at one input and zero at the other.
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