SBOU006 datasheet(20/33 Pages) TI2

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OPA3691

SBOS227E

www.ti.com

The total output spot noise voltage can be computed as the

square root of the sum of all squared output noise voltage

contributors. Equation 4 shows the general form for the

output noise voltage using the terms shown in Figure 12.

(4)

kTR NG

I R

kTR NG

(

) +

(

) +( ) +

Dividing this expression by the noise gain (NG = (1 + RF/RG))

will give the equivalent input referred spot noise voltage at

the noninverting input, as shown in Equation 5.

kTR

(

) ++ 







(5)

Evaluating these two equations for the OPA3691 circuit and

component values (see Figure 1) will give a total output spot

noise voltage of 8.08nV/

√Hz and a total equivalent input spot

noise voltage of 4.04nV/

√Hz. This total input-referred spot

noise voltage is higher than the 1.7nV/

√Hz specification for

the op amp voltage noise alone. This reflects the noise

added to the output by the inverting current noise times the

feedback resistor. If the feedback resistor is reduced in high

gain configurations (as suggested previously), the total input-

referred voltage noise given by Equation 5 will approach just

the 1.7nV/

√Hz of the op amp itself. For example, going to a

gain of +10 using RF = 182Ω will give a total input referred

noise of 2.1nV/

√Hz.

DC ACCURACY AND OFFSET CONTROL

A current-feedback op amp like the OPA3691 provides

exceptional bandwidth in high gains, giving fast pulse settling

but only moderate DC accuracy. The Electrical Characteris-

tics Table shows 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 effec-

tive with most voltage-feedback op amps, they do not gener-

ally reduce the output DC offset for wideband current-feed-

back op amps. Since the two input bias currents are unre-

lated 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 con-

figuration of Figure 1, using worst-case +25

°C input offset

voltage and the two input bias currents, gives a worst-case

output offset range equal to:

± (NG • V

OS(MAX)) + (IBN • RS/2 • NG) ± (IBI • RF)

where NG = noninverting signal gain

± (2 • 3.0mV) + (35µA • 25Ω • 2) ± (402Ω • 25µA)

±6mV + 1.75mV ± 10.05mV

= –14.3mV

→ +17.8mV

DISABLE OPERATION

The OPA3691 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 OPA3691 will operate normally.

To disable, the control pin must be asserted low. Figure 13

shows a simplified internal circuit for the disable control

feature.

FIGURE 12. Op Amp Noise Analysis Model.

4kT

1/3

OPA3691

4kT = 1.6E – 20J

at 290

°K

4kTR

√

4kTR

√

FIGURE 13. Simplified Disable Control Circuit.

25k

Ω

110k

Ω

15k

Ω

Control

–V

DIS

In normal operation, base current to Q1 is provided through

the 110k

Ω resistor while the emitter current through the 15kΩ

resistor sets up a voltage drop that is inadequate to turn on

the two diodes in Q1’s emitter. As V

DIS

is pulled low,

additional current is pulled through the 15k

Ω resistor eventu-

ally turning on these 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 approximately 0V.

This shuts off the collector current out of Q1, turning the

amplifier off. The supply current in the disable mode is that

only 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 OPA3691 is operating in a gain of +1,

this will show a very high impedance (4pF || 1M

Ω) 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

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