OPA890ID datasheet(21/34 Pages) NSC

21 / 34 page

4kT

OPA890

4kT=1.6E

20J

at290 K

4kTR

) IBNRS

) 4kTRS NG

2 ) (IBIRF)

) 4kTRFNG

) IBNRS

) 4kTRS )

BIRF

)

4kTR

OPA890

www.ti.com

SBOS369B – MAY 2007 – REVISED DECEMBER 2009

DC ACCURACY AND OFFSET CONTROL

The

balanced

input

stage

wideband

voltage-feedback op amp allows good output dc

accuracy in a wide variety of applications. The

power-supply current trim for the OPA890 gives even

tighter control than comparable amplifiers. Although

the high-speed input stage does require relatively

high input bias current (+25°C worst case, 1.6

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

Figure 52. Op Amp Noise Analysis Model

source resistances appearing at the two inputs. This

matching reduces the output dc error resulting from

The total output spot noise voltage can be computed

the input bias currents to the offset current times the

as the square root of the sum of all squared output

feedback resistor. Evaluating the configuration of

noise voltage contributors. Equation 4 shows the

Figure 46, and using worst-case +25°C input offset

general form for the output noise voltage using the

voltage and current specifications, gives a worst-case

terms shown in Figure 52.

output offset voltage equal to:

±(NG × VOS(MAX)) ± (RF × IOS(MAX))

= ±(2 × 5mV) ± (750

Ω × 0.35μA)

(4)

= ±11.3mV

Dividing this expression by the noise gain [NG = (1 +

RF/RG)] gives the equivalent input-referred spot noise

with NG = noninverting signal gain

voltage at the noninverting input, as shown in

A fine-scale output offset null or dc operating point

Equation 5.

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

(5)

resistor. In selecting an offset trim method, one key

consideration is the impact on the desired signal path

Evaluating these two equations for the OPA890

frequency response. If the signal path is intended to

circuit and component values (see Figure 46) gives a

be noninverting, the offset control is best applied as

total output spot noise voltage of 17.4nV/

√Hz and a

an inverting summing signal to avoid interaction with

total equivalent input spot noise voltage of 8.7nV/

√Hz.

the signal source. If the signal path is intended to be

This total includes the noise added by the bias

inverting,

applying

the

offset

control

the

current

cancellation

resistor

(175

Ω)

the

noninverting input may be considered. However, the

noninverting input. This total input-referred spot noise

dc offset voltage on the summing junction will set up

voltage is only slightly higher than the 8nV/

√Hz

a dc current back into the source that must be

specification for the op amp voltage noise alone. This

considered. Applying an offset adjustment to the

result will be the case, as long as the impedances

inverting op amp input can change the noise gain and

appearing at each op amp input are limited to the

frequency

response

flatness.

For

dc-coupled

previously recommend maximum value of 350

Ω.

inverting amplifier, see Figure 53 for one example of

Keeping both (RF || RG) and the noninverting input

an offset adjustment technique that has minimal

source impedance less than 350

Ω satisfies both

impact on the signal frequency response. In this

noise

and

frequency

response

flatness

case, the dc offsetting current is brought into the

considerations. Because the resistor-induced noise is

inverting input node through resistor values that are

relatively negligible, additional capacitive decoupling

much larger than the signal path resistors. This

across the bias current cancellation resistor (RB) for

configuration ensures that the adjustment circuit has

the inverting op amp configuration of Figure 51 is not

minimal effect on the loop gain and thus, the

required.

frequency response.

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