OPA692 datasheet(24/31 Pages) BURR-BROWN

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(NG

)+(I

R /2

NG)

R )

W ´

= (2

3.5mV)+(35 A

(50 A

300 )

= 7mV

1.75mV

15mV

= 23.75mV

E =

+(I

R ) +4kTR

NG +(I R ) +4kTR NG

DISABLE OPERATION

E =

+(I

R ) +4kTR

S +

4kTR

I R

DC ACCURACY AND OFFSET CONTROL

25kW

110kW

15kW

Control

-VS

DIS

OPA3693

SBOS353 – DECEMBER 2006

The total output spot noise voltage can be computed

as the square root of the sum of all squared output

noise voltage contributors. Equation 1 shows the

general form for the output noise voltage using the

terms shown in Figure 57.

where NG = noninverting signal gain.

Minimizing the resistance seen by the noninverting

input also minimizes the output dc error. For

Dividing this expression through by noise gain (NG =

improved dc precision in a wideband low-gain

1 + RF/RG) gives the equivalent input-referred spot

amplifier, consider the OPA842 where a bipolar input

noise voltage at the noninverting input, as shown in

is acceptable (low source resistance) or the OPA656

Equation 2.

where a JFET input is required.

The OPA3693 provides an optional disable feature

Evaluating

the

output

noise

and

input

noise

that can be used to reduce system power. If the VDIS

expressions

for

the

two

noninverting

gain

control

left

unconnected,

the

OPA3693

configurations, and with two different values for the

operates normally. This shutdown is intended only as

noninverting source impedance, gives output and

a power-savings feature. Forward path isolation

input-referred spot noise voltages of Table 3.

when disabled is very good for small signals for

gains of +1 or +2. Large-signal isolation is not

Table 3. Total Output and Input-Referred Noise

ensured. Using this feature to multiplex two or more

outputs together is not recommended. Large signals

OUTPUT SPOT

TOTAL INPUT

NOISE

SPOT NOISE

applied to the disabled output stages can turn on

CONFIGURATION

(

Ω)

EO (nV/√Hz)

EN (nV/√Hz)

parasitic devices degrading signal linearity for the

G = +2 (Figure 42)

8.3

4.15

desired channel.

G = +2 (Figure 42)

300

Turn-on time is very quick from the shutdown

G = +1 (Figure 43)

7.3

condition (typically < 60ns). Turn-off time strongly

G = +1 (Figure 43)

300

9.2

depends on the selected gain configuration and load,

but is typically 3

µs for the circuit of Figure 42.

The output noise is being dominated by the inverting

current noise times the internal feedback resistor.

To shutdown, the control pin must be asserted low.

This gives a total input-referred noise voltage that

This logic control is referenced to the positive supply,

exceeds the 1.8nV voltage term for the amplifier

as the simplified circuit of Figure 58 shows.

itself.

A current-feedback op amp such as the OPA3693

provides exceptional bandwidth and slew rate giving

fast pulse settling but only moderate dc accuracy.

The Electrical Characteristics show an input offset

voltage comparable to high-speed voltage-feedback

amplifiers. However, the two input bias currents are

somewhat higher and are unmatched. Whereas bias

current cancellation techniques are very effective

with most voltage-feedback op amps, they do not

generally reduce the output dc offset for wideband

current-feedback op amps. Since the two input bias

currents are unrelated in both magnitude and

polarity, matching the source impedance looking out

of each input to reduce their error contribution to the

output is ineffective. Evaluating the configuration of

Figure 58. Simplified Disable Control Circuit

Figure 42, using worst-case +25

°C input offset

voltage and the two input bias currents, gives a

In normal operation, base current to Q1 is provided

worst-case output offset range equal to:

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 the Q1

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