3

®

DEM-OPA68xU

OPA681U—Pin 8 disables the output when low. Table VIII

OPA682U—The OPA682U uses internal resistors to set the

gain at +1, +2, or –1. Pin 8 disables the output when low.

Table VIII shows different ways to set up pin 8’s voltage

except for the changes shown in Table IX.

OPA689U—This VLA™ is the high gain-stable version of

the OPA688U. Set up this op amp the same as the OPA688U,

except for the changes shown in Table XII.

DUAL SUPPLY

DUAL SUPPLY

SINGLE SUPPLY

COMPONENT

(G = +2)

(G = –1)

(G = +1)

Open

0

Ω

Open

0

Ω

Open

Open

Open

Open

Open

TABLE IX. OPA682U Changes.

OPA686U—Use Table X for the higher gains needed by

this part, and Table IV for the other component values. Extra

care is needed to ensure pin 8 is not connected (see Table I).

DUAL SUPPLY

DUAL SUPPLY

SINGLE SUPPLY

COMPONENT

(G = +10)

(G = –10)

(G = +20)

49.9

Ω

Open

49.9

Ω

20.0

Ω

Open

10.0

Ω

Open

10.0

Ω

Open

Open

49.9

Ω

Open

49.9

Ω

Open

26.1

Ω

453

Ω

499

Ω

499

Ω

TABLE X. OPA686U Changes.

OPA688U—This VLA™ (Voltage Limiting Amplifier) has

voltage swing. Table XI shows different ways to set up pin

Table IV for the other components.

Note that this board would require modification for the

single-supply circuit shown in the OPA688 data sheet. In

DUAL SUPPLY

DUAL SUPPLY

SINGLE SUPPLY

COMPONENT

(G = +2)

(G = –1)

(G = +1)

3.01k

Ω

3.01k

Ω

549

Ω

1.91k

Ω

1.91k

Ω

1.58k

Ω

3.01k

Ω

3.01k

Ω

Open

1.91k

Ω

1.91k

Ω

549

Ω

Open

Open

1.58k

Ω

0.1

µF

0.1

µF

0.1

µF

TABLE XI. Limiting Pins—OPA68xU.

DUAL SUPPLY

DUAL SUPPLY

SINGLE SUPPLY

COMPONENT

(G = +6)

(G = –6)

(G = +6)

49.9

Ω

82.5

Ω

49.9

Ω

100

Ω

Open

100

Ω

Open

130

Ω

Open

Open

124

Ω

Open

150

Ω

Open

150

Ω

750

Ω

750

Ω

750

Ω

TABLE XII. OPA689U Changes.

BOARD LAYOUT

This demonstration board is a two layer PCB. It uses a

ground plane on the bottom and signal and power traces on

the top. The ground plane has been opened up around op

amp pins sensitive to capacitive loading. Power supply

traces are laid out to keep current loop areas to a minimum.

The SMA (or SMB) connectors may be mounted either

vertically or horizontally.

The location and type of capacitors used for power-supply

bypassing are crucial to high frequency amplifiers. The

pins 7 and 4 on your PCB, and may be shared with other

amplifiers.

See the individual op amp data sheet for more information

on proper board layout techniques, and component selec-

tion.

MEASUREMENT TIPS

This demonstration board and the component values shown

are designed to operate in a 50

Ω environment. Most data

sheet plots are obtained this way. Change the component

values for different input and output impedance levels.

Do not use high impedance probes; they represent a heavy

capacitive load to the op amps, and will alter their response.

Instead, use low impedance (

≤ 500Ω) probes with adequate

bandwidth. The probe input capacitance and resistance set

an upper limit on the measurement bandwidth. If a high

impedance probe must be used, place a 100

Ω resistor on the

probe tip to isolate its capacitance from the circuit.