REV. D

AD844

–11–

Noise

Noise sources can be modeled in a manner similar to the dc bias

V

((Inp R

R1

R2









2

+ (Inn R1)

2

Overall noise can be reduced by keeping all resistor values to a

minimum. With typical numbers, R1 = R2 = 1 k

Vn = 2 nV/

calculates to 12 nV/

√Hz. The current noise is dominant in

this case, as it will be in most low gain applications.

Video Cable Driver Using

5 Volt Supplies

The AD844 can be used to drive low impedance cables. Using

±5 V supplies, a 100 Ω load can be driven to ± 2.5 V with low

distortion. Figure 11a shows an illustrative application which

provides a noninverting gain of 2, allowing the cable to be

reverse-terminated while delivering an overall gain of +1 to the

load. The –3 dB bandwidth of this circuit is typically 30 MHz.

Figure 11b shows a differential gain and phase test setup. In

video applications, differential-phase and differential-gain

characteristics are often important. Figure 11c shows the varia-

tion in phase as the load voltage varies. Figure 11d shows the

gain variation.

50

3

2

+5V

–5V

7

6

4

2.2 F

2.2 F

50

300

300

50

Figure 11a. The AD844 as a Cable Driver

HP8753A

NETWORK

ANALYZER

HP11850C

SPLITTER

CIRCUIT

UNDER

TEST

HP3314A

STAIRCASE

GENERATOR

OUT

OUT

OUT

IN

RF OUT

R

EXT

TRIG

SYNC OUT

50

(TERMINATOR)

OUT

470

Figure 11b. Differential Gain/Phase Test Setup Figure

0.3

0.2

–0.3

018

90

36

54

72

0.1

0

–0.1

–0.2

IRE = 7.14mV

Figure 11c. Differential Phase for the Circuit of Figure 11a

0.06

0.04

–0.06

0

18

90

36

54

72

0.02

0

–0.02

–0.04

IRE = 7.14mV

Figure 11d. Differential Gain for the Circuit of Figure 11a

High Speed DAC Buffer

The AD844 performs very well in applications requiring

current-to-voltage conversion. Figure 12 shows connections for

use with the AD568 current output DAC. In this application the

bipolar offset is used so that the full-scale current is

±5.12 mA,

which generates an output of

± 5.12 V usingdecoupling and

grounding techniques to achieve the full 12-bit accuracy and

realize the fast settling capabilities of the system. The unmarked

capacitors in this figure are 0.1

µF ceramic (for the 1 kΩ appli-

cation resistor on the AD568. Figure 13 shows the full-scale

transient response. Care is needed in power supply example,

AVX Type SR305C104KAA), and the ferrite inductors should

be about 2.5

µH (for example, Fair-Rite Type 2743002122).

The AD568 data sheet should be consulted for more complete

details about its use.

Applications–