®

OPA642

13

FIGURE 8. DC Coupled, Inverting Gain of –2, with Output

Offset Adjustment.

closest integer for minimum noise figure. This optimum

turns ratio is calculated by

This optimum will depend strongly on the amplifier and

configuration selected.

the offset control is best applied as an inverting summing

signal. If the signal path is intended to be inverting, applying

the offset control to the non-inverting input can be consid-

ered. For a DC coupled signal, the DC offset signal can, in

some configurations, set up a DC current back into the

source that must be considered. An adjustment placed on the

inverting op amp input can also change the noise gain and

frequency response flatness. Figure 8 shows one example of

an offset adjustment for a DC coupled signal path that will

have minimum impact on the signal frequency response. In

this case, the input is brought in to an inverting gain resistor

with the DC adjustment an additional current summed into

the inverting node. The resistor network setting this current

is much larger than the signal path resistors. This will insure

that this adjustment has minimal impact on the loop gain and

hence the frequency response.

FIGURE 7. Reduced Noise Figure Circuit.

DC OFFSET CONTROL

The OPA642 can provide excellent DC signal accuracy due

to its high open-loop gain, high common-mode rejection,

high power supply rejection, and low input offset voltage

and bias current offset errors. The high grade (B) version of

any package type provides less than 1mV input offset

voltage. To take full advantage of this low input offset

voltage, a careful attention to input bias current cancellation

is also required. The high speed input stage for the OPA642

has a relatively high input bias current (25

µA typ into the

pins) but with a very close match between the two input

currents—typically 100nA input offset current. The total

output offset voltage may be considerably reduced by match-

ing the source impedances looking out of the two inputs. For

example, one way to add bias current cancellation to the

circuit of Figure 1 would be to insert a 175

Ω series resistor

into the non-inverting input from the 50

Ω terminating resis-

tor. When the 50

Ω source resistor is DC coupled, this will

increase the source impedance for the non-inverting input

bias current to 200

Ω. Since this is now equal to the imped-

will cancel the gains for the bias currents to the output

leaving only the offset current times the feedback resistor as

a residual DC error term at the output. Using a 402

Ω

feedback resistor, this output error will now be less than 3

µA

• 402

Ω = 1.2mV.

A fine scale output offset null, or DC operating point

adjustment, is sometimes required. Numerous techniques

are available for introducing a DC offset control into an op

amp circuit. Most of these techniques eventually reduce to

setting up a DC current through the feedback resistor. One

key consideration to selecting a technique is to insure that it

has a minimal impact on the desired signal path frequency

response. If the signal path is intended to be non-inverting,

R

F

1k

Ω

±200mV Output Adjustment

= –

= –2

Supply Decoupling

Not Shown

5k

Ω

5k

Ω

328

Ω

0.1µF

R

G

500

Ω

V

I

20k

Ω

10k

Ω

0.1µF

–5V

+5V

OPA642

+5V

–5V

V

O

V

O

V

I

R

F

R

G

THERMAL ANALYSIS

The OPA642 will not require heatsinking under most oper-

ating conditions. Maximum desired junction temperature

will set a maximum allowed internal power dissipation as

described below. In no case should the maximum junction

temperature be allowed to exceed 175

°C.

Operating junction temperature (T

is given by

Quiescent power is simply the specified no-load supply

will depend on the required output signal and load but

would, for a grounded resistive load, be at a maximum when

the output is fixed at a voltage equal to 1/2 either supply

voltage (for equal bipolar supplies). Under this condition

P

loading.

Eq. 4

N

E

()

()









402

Ω

50

Ω

A

Supply Decoupling

Not Shown

6.3dB

Noise Figure

R

1:7

OPA642

402

Ω

2.4k

Ω

50

Ω

Load