SPT561AIJ Просмотр технического описания (PDF) - Signal Processing Technologies
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SPT561AIJ Datasheet PDF : 12 Pages
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This, and an expression for the external Cx without the
intermediate steps are shown below.
Cx
=
10 Ct
10 − Ct
or
Cx =
1
Ro
pF
− 0.08
300
1−
2
Rg
The plot in Figure 6 shows the required Cx vs. gain for
several desired output impedances using the equations
shown above. Note that for lower Ro’s, Cx can get very
large. But, since the total compensation is actually the
series combination of Cx and 10pF, going to very high
Cx’s is increasingly ineffective as the total compensation
is only slightly changed. This, in part, sets the lower limits
on allowable Ro.
20
18
Maximally Flat Response
into a Matched Load
16
14
12
Ro = 50Ω
10
8
6
Ro = 75Ω
4
2
Ro = 100Ω
0
5 10 15 20 25 30 35 40 45 50 55
No Load Voltage Gain
Figure 6: External Compensation Capacitance (Cx)
A 0% small signal overshoot response can be achieved
by increasing Cx slightly from the maximally flat value.
Note that this applies only for small signals due to slew
rate effects coming into play for large, fast edge rates.
Beyond the nominal compensation values developed
thus far, this external Cx provides a very flexible means
for tailoring the frequency response under a wide variety
of gain and loading conditions. It is oftentimes useful to
use a small adjustable cap in development to determine
a Cx suitable to the application, then fixing that value for
production. An excellent 5pF to 20pF trimmer cap for this
is a Sprague-Goodman part #GKX20000.
When the SPT561 is used to drive a capacitive load,
such as an ADC or SAW device, the load will act to
compensate the response along with Cx. Generally,
considerably lower Cx values are required than the
earlier development would indicate. This is advanta-
geous in that a low Ro would be desired to drive a
capacitive load which, without the compensating effect of
load itself, would otherwise require very large Cx values.
Gain and Output Impedance Range
Figure 7 shows a plot of the recommended gain and
output impedances for the SPT561. Operation outside of
this region is certainly possible with some degradation in
performance. Several factors contribute to set this range.
At very low output impedances, the required value of
feedback resistor becomes so low as to excessively load
the output causing a rapid degradation in distortion.
The maximum Ro was set somewhat arbitrarily at 200Ω.
This allows the SPT561 to drive into a 2:1 step down
transformer matching to a 50Ω load. (This offers
some advantages from a distortion standpoint. See Appli-
cation Note SPT-01.
100
90
80
70
60
50
40
30
20
10
0
0
Low Rf or Rg Region
Recommended
Region
High Noise Region
20 40 60 80 100 120 140 160 180 200
Output Impedance (Ω)
Figure 7: Recommended Gain and
Output Impedance Range
For a given Ro, the minimum gain shown in Figure 7 has
been set to keep the equivalent input noise voltage less
than 4nV/√Hz. Generally, the equivalent input noise
voltage decreases with higher signal gains. The high gain
limit has been set by targeting a minimum Rg of 10Ω or a
minimum Rf of 100Ω.
Amplifier Configurations
(Additional discussion in Application Note SPT-01.) The
SPT561 is intended for a fixed, non-inverting, gain con-
figuration as shown in Figure 1. Due to its low internal
forward gain, the inverting node does not present a low
impedance, or virtual ground, node. Hence, in an invert-
ing configuration, the signal’s source impedance will see
a finite load whose value depends on the output loading.
Inverting mode operation can be best achieved using a
wideband, unity gain buffer with low output impedance, to
isolate the source from this varying load. A DC level can,
however, be summed into the inverting node to offset the
output either for offset correction or signal conditioning.
SPT Application Note SPT-01 describes this and a
composite amplifier structure that enhances the DC and
gain accuracy characteristics of the SPT561.
Accuracy Calculations
Several factors contribute to limit the achievable SPT561
accuracy. These include the DC errors, noise effects, and
SPT561
9
10/9/98
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