ISL70517SEH Datasheet, PDF (22/27 Pages) Intersil Corporation – Precision test and measurement

ISL70517SEH
Estimating Amplifier DC and
Noise Performance
The gain resistor ohmic values and ratios are all that are required
to estimate DC offset and noise. The following sections illustrate
methods to calculate DC offset and noise performance. These
estimates are useful for optimizing resistor values for noise and
DC offset.
Calculating DC Offset Voltage
Output offset voltage, like output noise, has several contributors.
Also similar to output noise, the major offset contributor depends
on the gain configuration. In high-gain, V_OS(I) dominates, while in
low-gain, offset due to I_ERR dominates.
The summation of DC offsets to arrive at a total DC offset error is
performed in two ways. Equation 13 is a simple addition of the
DC offsets appearing at the output and is useful when defining
the minimum to maximum range of offset that can be expected.
The drawback is that the result defines the corner of the corners
of the error box and is not a typical value given that these sources
are uncorrelated.
V_OSRTO= A_V  V_OS(IN) + V_{OSFB} + I_ERR  R_f (EQ. 13)
Equation 14 expresses the total DC error as the RMS, or square
root of the sum of the squares to provide an estimate of a typical
value.
V_OS(RTO)TYP = √[(A_V × V_OS(IN))² + (V_OS(FB))² + (I_ERR × R_f)²] (EQ. 14)
Equation 15 converts the output offset error range to an input
referred error range [V_OS(RTI)] and enables a comparison with
the DC component of the input signal.
V_OSRTI= V_OS(IN) + V_OS(FB)  A_V + I _ERR  R_FB   A_V
(EQ. 15)
Similarly, Equation 16 shows the typical DC offset value referred
to the input.
V_OS(RTI)TYP = √[V_OS(IN))² + (V_OS(FB)/A_V)² + (I_ERR × R_FB)_/A_V)²]
(EQ. 16)
These results are summarized in Table 2.
Calculating Noise Voltage
The calculation of noise spectral density at the output [e_N(RTO)]
from all noise sources is given by Equation 17.
e_N(RTO) = √[(A_V × e_N(I))² + (2 × A_V × i_N(I) × 500Ω)² +
(EQ. 17)
(A_V)² x (4kT × R_IN) + (4kT × R_f) + (R_f × i_N(I_ERR))²+(e_N(FB))²]
Equation 18 converts the output noise to the input referred value
when evaluating the input signal-to-noise ratio.
e_NRTI= e_NRTO  A_V
(EQ. 18)
Table 3 provides examples of the noise contribution of each
source by circuit gain and output voltage span.
In a high-gain configuration, the input noise is the dominant
noise source. In a low-gain configuration, the noise voltage from
the product of the internal noise current, I_N(err), and the feedback
resistor, R_FB, dominates. The contribution of the internal noise
current, I_N(err), increases in proportion to R_FB, but the
corresponding increase in output voltage with R_FB keeps the ratio
of this noise voltage to an output voltage constant.
TABLE 2. COMPUTING TYPICAL OUTPUT OFFSET VOLTAGE RANGES
R_IN
A_V
V_O(LIN)
(kΩ)
1
±2.5
30.1
R_f
(kΩ)
30.1
A_V x V_OS(I)
(µV)
(Note 17)
±30
V_OS(FB)
(µV)
(Note 17)
±400
I_ERR (5nA)
x R_FB
(µV)
(Note 17)
±150
V_OS(RTO)
(µV)
Equation 13
V_OS (RTI)
(µV)
Equation 15
TYPICAL
V_OS(RTO)
(µV)
Equation 14
TYPICAL
V_OS(RTI)
(µV)
Equation 16
±580
428
1
±10
121
121
±30
±400
±600
±1030
722
100 ±2.5 0.301 301
±3000
±400
±150
±3550
±3005
3030
3000
100 ±10
1.21
121
±3000
±400
±600
±4000
±3010
3085
3000
NOTE:
17. Chosen for illustration purposes and does not reflect actual device performance.
TABLE 3. 1kHz INPUT NOISE AND THERMAL NOISE CONTRIBUTIONS
R_IN R_FB
A_V (kΩ) (kΩ)
1 30.1 30.1
A_V x e_N(I)
(nV/√Hz)
8.6
2 x A_V x i_N(I)
x 500Ω
(nV/√Hz)
0.15
A_V x √(4kT
x R_IN)
(nV/√Hz)
22.3
√(4kT x R_FB)
(nV/√Hz)
22.3
R_FB x i_N(IERR)
(nV/√Hz)
90
e_N(FB)
(nV/√Hz)
8.6
1 121 121
8.6
0.15
44.6
44.6
360
8.6
100 0.301 301
860
15
223
22.3
90
8.6
100 1.21 121
860
15
446
44.6
360
8.6
NOTE:
18. e_N and i_N values are chosen for illustration purposes and may not reflect actual device performance.
e_N (RTO)
OUTPUT
REFERRED NOISE
(nV/√Hz)
e_N (RTI) INPUT
REFERRED
NOISE
(nV/√Hz)
96
366
898
8.9
1035
10
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December 15, 2016