OPA684IDBVT datasheet(19/25 Pages) TI

19 / 25 page

OPA684

SBOS219A

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 3 shows the general form for the

output noise voltage using the terms shown in Figure 13.

(3)

kTR

I R

kTR G

BN S

BI F

(

) +









(

) +

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

will give the equivalent input referred spot noise voltage at

the non-inverting input, as shown in Equation 4.

(4)

kTR

I R

kTR

BN S

BI F

(

) +









Evaluating these two equations for the OPA684 circuit and

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

noise voltage of 13.3nV/

√Hz and a total equivalent input spot

noise voltage of 6.7nV/

√Hz. This total input referred spot

noise voltage is higher than the 3.7nV/

√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. As the gain is increased, this fixed output

noise power term contributes less to the total output noise

and the total input referred voltage noise given by Equation

3 will approach just the 3.7nV/

√Hz of the op amp itself. For

example, going to a gain of +20 in the circuit of Figure 1,

adjusting only the gain resistor to 52.3

Ω, will give a total input

referred noise of 3.9nV/

√Hz . A more complete description of

op amp noise analysis can be found in the TI application note

AB-103, “Noise Analysis for High Speed Op Amps”

(SBOA066), located at www.ti.com.

DC ACCURACY AND OFFSET CONTROL

A current-feedback op amp like the OPA684 provides excep-

tional bandwidth in high gains, giving fast pulse settling but

only moderate DC accuracy. The Electrical Characteristics

show an input offset voltage comparable to high slew rate

voltage-feedback amplifiers. The two input bias currents,

however, are somewhat higher and are unmatched. Whereas

bias current cancellation techniques are very effective with

most voltage-feedback op amps, they do not generally re-

duce 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 1, using worst case +25

°C input offset voltage and the

two input bias currents, gives a worst case output offset

range equal to:

±(G

N • VOS) + (IBN • RS/2 • GN) ± (IBI • RF)

±(2 • 3.5mV) ± (10µA • 25Ω • 2) ± (1kΩ • 16µA)

±7mV + 0.5mV ± 16mV

±23.5mV

where GN = non-inverting signal gain

While the last term, the inverting bias current error, is

dominant in this low-gain circuit, the input offset voltage will

become the dominant DC error term as the gain exceeds

5V/V. Where improved DC precision is required in a high-

speed amplifier, consider the OPA642 single and OPA2822

dual voltage-feedback amplifiers.

DISABLE OPERATION

The OPA684 provides an optional disable feature that may

be used to reduce system power when amplifier operation is

not required. If the VDIS control pin is left unconnected, the

OPA684 will operate normally. To disable, the VDIS control

pin must be asserted LOW. Figure 14 shows a simplified

internal circuit for the disable control feature.

In normal operation, base current to Q1 is provided through

the 250k

Ω resistor, while the emitter current through the

40k

Ω resistor sets up a voltage drop that is inadequate to

turn on the two diodes in Q1’s emitter. As VDIS is pulled LOW,

additional current is pulled through the 40k

Ω resistor eventu-

ally turning on these two diodes. At this point, any further

current pulled out of VDIS 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 only what is

required to operate the circuit of Figure 14.

25k

Ω

250k

Ω

40k

Ω

Control

–V

DIS

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

impedance state. If the OPA684 is operating at a gain of +1

(with a 1k

Ω feedback resistor still required for stability), it will

show a very high impedance (1.7pF || 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 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) giving relatively poor input to output

isolation.

FIGURE 14. Simplified Disable Control Circuit.

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