9

®

OPA4650

impossible to achieve a smooth, stable response. Best results

are obtained by soldering the part onto the board. If socket-

ing for the DIP package is desired, high frequency flush

mount pins (e.g., McKenzie Technology #710C) can give

good results.

The OPA4650 is nominally specified for operation using

±5V

power supplies. A 10% tolerance on the supplies, or an ECL

–5.2V for the negative supply, is within the maximum speci-

fied total supply voltage of 11V. Higher supply voltages can

break down internal junctions possibly leading to catastrophic

failure. Single supply operation is possible as long as com-

mon mode voltage constraints are observed. The common

mode input and output voltage specifications can be inter-

preted as a required headroom to the supply voltage. Observ-

ing this input and output headroom requirement will allow

non-standard or single supply operation. Figure 1 shows one

approach to single-supply operation.

OFFSET VOLTAGE ADJUSTMENT

One simple way to null the initial offset voltage while

retaining the low offset drift of the OPA4650 is shown in

Figure 2. The 20k

Ω potentiometer and the 47kΩ series

summed into the inverting node. The 0.1

µF capacitor keeps

high-frequency power supply noise from coupling into the

signal path. Although the initial offset will be nulled to zero

with this technique, issues of temperature drift must also be

considered. The additional resistor R

assumed to be negligible in this calculation). This will

eliminate the first-order offset drift due to input bias current

leaving only the input offset current (I

ESD PROTECTION

ESD damage has been a well recognized source of degrada-

tion for MOSFET type circuits, but any semiconductor

device can be vulnerable to damage. This becomes more of

an issue for very high speed processes like that used for the

FIGURE 1. Single Supply Operation.

FIGURE 2. Offset Voltage Trim.

OPA4650. ESD damage can cause subtle changes in ampli-

fier input characteristics without necessarily destroying the

device. In precision operational amplifiers, this may cause a

noticeable degradation of offset voltage and drift. ESD

handling precautions are strongly recommended when han-

dling the OPA4650.

OUTPUT DRIVE CAPABILITY

The OPA4650 has been optimized to drive 75

Ω and 100Ω

resistive loads. The device can drive 1Vp-p into a 75

Ω

load. This high output drive capability makes the OPA4650

an ideal choice for a wide range of RF, IF and video

applications. In many cases, additional buffer amplifiers

are unnecessary.

Many demanding high speed applications, such as driving

Analog-to-Digital converters, require op amps with low

wideband output impedance. For example, low output imped-

ance is essential when driving the signal-dependent capaci-

tance at the input of a flash A/D converter. As shown in

Figure 3, the OPA4650 maintains very low closed-loop

output impedance over frequency. Closed-loop output imped-

ance increases with frequency since loop gain is decreasing.

SMALL-SIGNAL OUTPUT IMPEDANCE

vs FREQUENCY

Frequency (Hz)

1k

100

10

1

0.1

0.01

10k

100k

1M

100M

10M

G = +1

FIGURE 3. Small-Signal Output Impedance vs Frequency.

R

2

OPA4650

R

(1)

R

1

R

TRIM

+V

s

–V

S

20k

Ω

V

Output Trim Range

+V

S

to –V

S

NOTE: (1) R

due to input bias currents.

≅

R

TRIM

47k

Ω

R

2

R

2

R

TRIM

0.1µF

R

G

402

Ω

1/4

OPA4650

V

AC

R

F

402

Ω

R

L

+V

S

+V

S

V

S

2

R

OUT

V

S

2

V

+ A

A

R

F

R

G