THS7001, THS7002

70-MHz PROGRAMMABLE-GAIN AMPLIFIERS

SLOS214B – OCTOBER 1998 – REVISED AUGUST 1999

18

POST OFFICE BOX 655303

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:

NF

+ 10log 1 )

e

n

2

) IN )

R

S

2

4kTR

S

Figure 54 shows the noise figure graph for the THS7001 and THS7002.

PREAMP NOISE FIGURE

vs

SOURCE RESISTANCE

14

12

10

8

6

4

10

100

1k

Source Resistance –

Ω

2

10k

16

f = 10 kHz

TA = 25 _C

0

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.