OPA690 datasheet(18/22 Pages) TI

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OPA690

SBOS223A

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LOW, additional current is pulled through the 15k

Ω resistor,

eventually turning on those two diodes (

≈75µA). At this point,

any further current pulled out of V

DIS goes through those

diodes holding the emitter-base voltage of Q1 at approxi-

mately 0V. This shuts off the collector current out of Q1,

turning the amplifier off. The supply current in the disable

mode are only those required to operate the circuit of Figure

13. Additional circuitry ensures that turn-on time occurs

faster than turn-off time (make-before-break).

When disabled, the output and input nodes go to a high

impedance state. If the OPA690 is operating in a gain of +1,

this will show a very high impedance at the output and

exceptional signal isolation. If operating at a gain greater

than +1, the total feedback network resistance (RF + RG) will

appear as the impedance looking back into the output, but

the circuit will still show very high forward and reverse

isolation. If configured as an inverting amplifier, the input and

output will be connected through the feedback network

resistance (RF + RG) and the isolation will be very poor as a result.

One key parameter in disable operation is the output glitch

when switching in and out of the disabled mode. Figure 14

shows these glitches for the circuit of Figure 1 with the input

signal at 0V. The glitch waveform at the output pin is plotted

along with the DIS pin voltage.

A fine-scale output offset null, or DC operating point adjust-

ment, 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 consid-

ered. However, the DC offset voltage on the summing

junction will set 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 12 shows one example of an offset adjust-

ment technique that has minimal impact on the signal fre-

quency 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 will

insure that the adjustment circuit has minimal effect on the

loop gain and hence the frequency response.

FIGURE 13. Simplified Disable Control Circuit.

FIGURE 12. DC-Coupled, Inverting Gain of –2, with Offset

Adjustment.

FIGURE 14. Disable/Enable Glitch.

Ω

±200mV Output Adjustment

= –

= –2

Supply Decoupling

Not Shown

Ω

328

Ω

0.1

µF

500

Ω

20k

Ω

10k

Ω

0.1

µF

–5V

+5V

OPA690

+5V

–5V

25k

Ω

110k

Ω

15k

Ω

Control

–V

DIS

DISABLE OPERATION

The OPA690 provides an optional disable feature that may

be used either to reduce system power or to implement a

simple channel multiplexing operation. If the DIS control pin

is left unconnected, the OPA690 will operate normally. To

disable, the control pin must be asserted LOW. Figure 13

shows a simplified internal circuit for the disable control

feature.

In normal operation, base current to Q1 is provided through

the 110k

Ω resistor, while the emitter current through the

15k

Ω resistor sets up a voltage drop that is inadequate to

turn on the two diodes in Q1’s emitter. As V

DIS is pulled

DISABLE/ENABLE GLITCH

Time (20ns/div)

–10

–20

–30

i = 0V

DIS

Output Voltage

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