THS7001_16 Datasheet, PDF (18/41 Pages) Texas Instruments – 70-MHz PROGRAMMABLE-GAIN AMPLIFIERS

THS7001, THS7002
ą
70ĆMHz PROGRAMMABLEĆGAIN AMPLIFIERS
ą
SLOS214C − OCTOBER 1998 − REVISED MARCH 2007
APPLICATION INFORMATION
noise calculations and noise figure (continued)
Because the dominant noise components are generally the source resistance and the internal amplifier noise
voltage, we can approximate noise figure as:
ǒ Ǔ _NF
+
ȱ
10log^{ȧȧȧȧȧ}1
ȧȡȢ
)
en
2
) ^ǒIN )
4 kTRS
Ȳ
RS^Ǔ2_{ȧȣȤȳȧȧȧȧȧ}
È´
Figure 54 shows the noise figure graph for the THS7001 and THS7002.
PREAMP NOISE FIGURE
vs
SOURCE RESISTANCE
16
f = 10 kHz
14 TA = 25 _C
12
10
8
6
4
2
0
10
100
1k
10k
Source Resistance − Ω
Figure 54. Noise Figure vs Source Resistance
optimizing frequency response for the preamplifiers
Internal frequency compensation of the THS7001 and THS7002 was selected to provide very wide bandwidth
performance and still maintain a very low noise floor. In order to meet these performance requirements, the
preamplifiers must have a minimum gain of 2 (−1). Because everything is referred to the noninverting terminal
of an operational amplifier, the noise gain in a G = −1 configuration is the same as a G = 2 configuration.
One of the keys of maintaining a smooth frequency response, and hence, a stable pulse response, is to pay
particular attention to the inverting terminal. Any stray capacitance at this node causes peaking in the frequency
response. There are two things that can be done to help minimize this effect. The first is to simply remove any
ground planes under the inverting terminal of the amplifier. This also includes the trace that connects to this
terminal. Additionally, the length of this trace should be minimized. The capacitance at this node causes a lag
in the voltage being fed back due to the charging and discharging of the stray capacitance. If this lag becomes
too long, the amplifier will not be able to correctly keep the noninverting terminal voltage at the same potential
as the inverting terminal’s voltage. Peaking and possibly oscillations can occur if this happens.
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