Datasheet

 
   
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SLOS274D − DECEMBER 1999 − REVISED JUNE 2001
11
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
theory of operation
The THS408x is a high-speed, operational amplifier configured in a voltage feedback architecture. It is built
using a 30-V, dielectrically isolated, complementary bipolar process with NPN and PNP transistors possessing
f
T
s of several GHz. This results in an exceptionally high performance amplifier that has a wide bandwidth, high
slew rate, fast settling time, and low distortion. A simplified schematic is shown in Figure 38.
IN− (2)
IN+ (3)
(6) OUT
(4) V
CC
−
(7) V
CC
+
Figure 38. THS4081 Simplified Schematic
noise calculations and noise figure
Noise can cause errors on very small signals. This is especially true when amplifying small signals, where
signal-to-noise ratio (SNR) is very important. The noise model for the THS408x is shown in Figure 39. This
model includes all of the noise sources as follows:
• e
n
= Amplifier internal voltage noise (nV/√Hz)
• IN+ = Noninverting current noise (pA/√Hz)
• IN− = Inverting current noise (pA/√Hz)
• e
Rx
= Thermal voltage noise associated with each resistor (e
Rx
= 4 kTR
x
)