7

LT1222

Output Clamping

Access to the internal compensation node at pin 5 allows

the output swing of the LT1222 to be clamped. An example

is shown on the first page of this data sheet. The compen-

sation node is approximately one diode drop above the

output and can source or sink 1.2mA. Back-to-back Schot-

tky diodes clamp pin 5 to a diode drop above ground so the

output is clamped to

±0.5V (the drop of the Schottkys at

1.2mA). The diode reference is bypassed for good AC

response. This circuit is useful for amplifying the voltage at

false sum nodes used in settling time measurements.

Capacitive Loading

The LT1222 is stable with capacitive loads. This is accom-

plished by sensing the load induced output pole and adding

compensation at the amplifier gain node. As the capacitive

load increases, both the bandwidth and phase margin

decrease. There will be peaking in the frequency domain as

shown in the curve of Frequency Response vs Capacitive

Load. The small-signal transient response will have more

overshoot as shown in the photo of the small-signal

response with 1000pF load. The large-signal response with

a 10,000pF load shows the output slew rate being limited

to 4V/

µs by the short-circuit current. The LT1222 can drive

coaxial cable directly, but for best pulse fidelity a resistor of

value equal to the characteristic impedance of the cable

(i.e., 75

Ω) should be placed in series with the output. The

other end of the cable should be terminated with the same

value resistor to ground.

Compensation

The LT1222 has a typical gain-bandwidth product of

500MHz which allows it to have wide bandwidth in high

gain configurations (i.e., in a gain of 100, it will have a

bandwidth of about 5MHz). For added flexibility the ampli-

fier frequency response may be adjusted by adding capaci-

tance from pin 5 to ground. The compensation capacitor

may be used to reduce overshoot, to allow the amplifier to

be used in lower noise gains, or simply to reduce band-

width. Table 1 shows gain and compensation capacitor

vresus – 3dB bandwidth, maximum frequency peaking and

small-signal overshoot.

Table 1

Max Peaking (dB)

Overshoot (%)

– 1

30

99

4.2

36

– 1

50

70

0.9

13

– 1

82

32

0

0

– 1

150

13

0

0

5

10

140

3.8

35

5

20

100

0

5

530

34

0

1

550

15

0

0

10

0

150

9.5

45

10

5

111

0.2

10

10

10

40

0

2

10

20

17

0

0

20

0

82

0.1

10

20

5

24

0

0

20

10

14

0

0

For frequencies < 10MHz the frequency response of the

amplifier is approximately:

f = 1/[2

The slew rate is affected as follows:

An example would be a gain of –10 (noise gain of 11) and

rate. It should be noted that the LT1222 is not stable in

feedback resistor. The 1k and input capacitance increase

the noise gain at frequency to aid stability.