Single/Triple, Low-Glitch, 250MHz, Current-

Feedback Amplifiers with High-Speed Disable

16

______________________________________________________________________________________

amplifiers achieve very high isolation from input to output

(65dB at 10MHz), and the outputs are placed into a high-

impedance state. These amplifiers achieve low switch-

ing-transient glitches (<45mVp-p) when switching

between enable and disable modes. Fast enable/disable

times (120ns/35ns), along with high off-isolation and low

switching transients, allow these devices to be used as

high-performance, high-speed multiplexers. This is

achieved by connecting the outputs of multiple amplifiers

together and controlling the DISABLE inputs to enable

one amplifier and disable all others. The disabled ampli-

fiers present a very light load (1µA leakage current and

3.5pF capacitance) to the active amplifier’s output. The

feedback network impedance of all the disabled ampli-

fiers must still be considered when calculating the total

load on the active amplifier output. Figure 1 shows an

application circuit using the MAX4188 as a 3:1 video mul-

tiplexer.

±5V supplies, the disable inputs are CMOS-logic com-

patible. The amplifiers default to the enabled mode if

the DISABLE pin is left unconnected. If the DISABLE

pin is left floating, take proper care to ensure that no

high-frequency signals are coupled to this pin, as this

may cause false triggering.

Applications Information

Theory of Operation

The MAX4188/MAX4189/MAX4190 are current-feedback

amplifiers, and their open-loop transfer function is

expressed as a transimpedance,

frequency behavior of the open-loop transimpedance is

similar to the open-loop gain of a voltage-mode feedback

amplifier. That is, it has a large DC value and decreases

at approximately 6dB per octave.

Analyzing the follower with gain, as shown in Figure 2,

yields the following transfer function:

≅

300

Ω.

bandwidth is essentially independent of closed-loop

V

V

OUT

IN

(

/

)

=

=

+

G

R

R

F

G

1

+1

+1

MAX4188

MAX4189

MAX4190

Figure 2. Current-Feedback Amplifier

AMP1

6

7

5

560

Ω

75

Ω

560

Ω

1.0

µF

1.0

µF

0.1

µF

4

11

+5V

-5V

0.1

µF

87

Ω

AMP2

13

14

12

9

10

DISABLE2

DISABLE3

DISABLE1

560

Ω

75

Ω

560

Ω

87

Ω

75

Ω

75

Ω

CABLE

8

1

2

3

560

Ω

75

Ω

560

Ω

87

Ω

MAX4188

AMP3

Figure 1. High-Speed 3:1 Video Multiplexer