OPA690 datasheet(25/35 Pages) TI

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DC ACCURACY AND OFFSET CONTROL

750W

±200mVOutputAdjustment

= 2

Power-supply

decouplingnotshown.

5kW

261W

0.1 F

375W

20kW

10kW

0.1 F

-5V

+5V

1/2

OPA2890

+5V

-5V

OS(MAX)

(NG

)

= (2

5mV)

(750

= 11.2mVwith (NG=noninvertingsignalgain)

OS(MAX)

1.6

DISABLE OPERATION (MSOP-10 Package

200kW

2MW

50kW

Control

-VS

DIS

OPA2890

SBOS364A – DECEMBER 2007 – REVISED DECEMBER 2007

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 OPA2890 gives

even tighter control than comparable amplifiers.

Although the high-speed input stage does require

relatively high input bias current (typically 5

µA out of

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

matching reduces the output dc error resulting from

the input bias currents to the offset current times the

feedback resistor. Evaluating the configuration of

Figure 49, and using worst-case +25

°C input offset

voltage and current specifications, gives a worst-case

output offset voltage equal to:

Figure 61. DC-Coupled, Inverting Gain of –2, with

Offset Adjustment

A fine-scale output offset null, or dc operating point

Only)

adjustment, is often required. Numerous techniques

are available for introducing dc offset control into an

The OPA2890IDGS provides an optional disable

op amp circuit. Most of these techniques eventually

feature that can be used either to reduce system

reduce to adding a dc current through the feedback

power or to implement a simple channel multiplexing

resistor. In selecting an offset trim method, one key

operation. If the DIS control pin is left unconnected,

consideration is the impact on the desired signal path

the OPA2890IDGS operates normally. To disable, the

frequency response. If the signal path is intended to

control pin must be asserted LOW. Figure 62 shows

be noninverting, the offset control is best applied as

a simplified internal circuit for the disable control

an inverting summing signal to avoid interaction with

feature.

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

inverting,

applying

the

offset

control

the

noninverting input may be considered. However, the

dc offset voltage on the summing junction sets up a

dc current back into the source that 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 61 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

configuration ensures that the adjustment circuit has

minimal effect on the loop gain and thus, the

frequency response.

Figure 62. Simplified Disable Control Circuit

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