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E

N +

E

NI

2

) IBNRS

2

) 4kTRS )

I

BI

R

F

NG

2

)

4kTR

F

NG

(6)

DC Accuracy and Offset Control

OPA698M

SGLS287A – MAY 2005 – REVISED MARCH 2006

Evaluating these two equations for the OPA698 circuit and component values (see Figure 47) gives a total output

spot noise voltage of 11.9 nV/

√Hz and a total equivalent input spot noise voltage of 6 nV/√Hz. This total

input-referred spot noise voltage is only slightly higher than the 5.6 nV/

√Hz specification for the op amp voltage

noise alone. This is the case as long as the impedances appearing at each op amp input is limited to a maximum

value of 300

Ω. Keeping both (R

F || RG) and the noninverting input source impedance less than 300 Ω satisfies

both noise and frequency response flatness considerations. Since the resistor-induced noise is relatively

negligible, additional capacitive decoupling across the bias current cancellation resistor (RT) for the inverting op

amp configuration of Figure 49 is not required, but is still desirable.

The balanced input stage of a wideband voltage feedback op amp allows good output dc accuracy in a large

variety of applications. The power-supply current trim for the OPA698 gives even tighter control than comparable

products. Although the high-speed input stage does require relatively high input bias current (typically ±8 µA at

each input terminal), the close matching between them may be used to reduce the output dc error caused by this

current. The total output offset voltage may be considerably reduced by matching the dc source resistances

appearing at the two inputs. This reduces the output dc error due to the input bias currents to the offset current

times the feedback resistor. Evaluating the configuration of Figure 47, using a worst-case +25°C input offset

voltage and current specifications, gives a worst-case output offset voltage equal to:

– (NG = noninverting signal gain)

± (NG × V

IO(MAX)) ± [RF x IIO(MAX)]

= ±(2 × 8 mV) ± (402

Ω × 3 µA)

= ±17.2 mV

A fine-scale output offset null or dc operating point adjustment is often required. Numerous techniques are

available for introducing dc offset control into an op amp circuit. Most of these techniques eventually reduce to

adding a dc current through the feedback resistor. In selecting an offset trim method, one key consideration is

the impact on the desired signal path frequency response. If the signal path is intended to be noninverting, the

offset control is best applied as an inverting summing signal to avoid interaction with the signal source. If the

signal path is intended to be inverting, applying the offset control to the noninverting input may be considered.

However, the dc offset voltage on the summing junction sets up a dc current back into the source which must be

considered. Applying an offset adjustment to the inverting op amp input can change the noise gain and

frequency response flatness. For a dc-coupled inverting amplifier, Figure 71 shows one example of an offset

adjustment technique that has minimal impact on the signal frequency response. In this case, the dc offsetting

current is brought into the inverting input node through resistor values that are much larger than the signal path

resistors. This insures that the adjustment circuit has minimal effect on the loop gain as well as the frequency

response.

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