OPA690 datasheet(23/27 Pages) TI

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OPA2613

SBOS249D − JUNE 2003− REVISED APRIL 2004

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DIFFERENTIAL NOISE PERFORMANCE

As the OPA2613 is used as a differential driver in xDSL

applications, it is important to analyze the noise in such a

configuration. Figure 14 shows the op amp noise model for

the differential configuration.

Driver

√4kTR

Figure 14. Differential Op Amp Noise Analysis

Model

As a reminder, the differential gain is expressed as:

+ 1 )

The output noise can be expressed as shown below:

O +

2 ) i

) 4kTR

) 2 i

) 2 4kTR

Dividing this expression by the differential noise gain

(GD = (1 + 2RF/RG)) gives the equivalent input referred

spot noise voltage at the noninverting input, as shown in

Equation 21.

i +

2 ) i

) 4kTR

) 2

IRF

) 2

4kTR

Evaluating these equations for the OPA2613 ADSL circuit

and component values of Figure 6 gives a total output spot

noise voltage of 23.3nV/

√Hz and a total equivalent input

spot noise voltage of 3.2nV/

√Hz.

In order to minimize the output noise due to the

noninverting input bias current noise, it is recommended to

keep the noninverting source impedance as low as

possible.

DC ACCURACY AND OFFSET CONTROL

The OPA2613 can provide excellent DC signal accuracy

due to its high open-loop gain, high common-mode

rejection, high power-supply rejection, and low input offset

voltage and bias current offset errors. To take full

advantage of the low input offset voltage (

±1.0mV

maximum at 25

°C), careful attention to input bias current

cancellation is also required. The high-speed input stage

for the OPA2613 has relatively high input bias current (6

µA

typical into the pins) but with a very close match between

the two input currents, typically 50nA input offset current.

The total output offset

voltage

may

reduced

considerably by matching the source impedances looking

out of the two inputs. For example, one way to add bias

current cancellation to the circuit of Figure 1 would be to

insert a 175

Ω series resistor into the noninverting input

from the 50

Ω terminating resistor. If the 50Ω source

resistor is DC-coupled, this will increase the source

impedance for the noninverting input bias current to 200

Ω.

Since this is now equal to the impedance looking out of the

inverting input (RF || RG), the circuit will cancel the bias

current effects, leaving only the offset current times the

feedback resistor as a residual DC error term at the output.

Evaluating the configuration of Figure 1 adding a 175

Ω in

series with the noninverting input pin, using worst-case

+25

°C input offset voltage and the two input bias currents,

gives a worst-case output offset range equal to:

VOFF = ± (NG × VOS(MAX)) ± (IOS × RF)

where NG = noninverting signal gain

± (2 × 1.0mV) ± (402Ω × 300nA)

±2.0mV ± 0.12mV

VOFF = ±2.12mV

THERMAL ANALYSIS

Due to the high output power capability of the OPA2613,

heat-sinking or forced airflow may be required under

extreme operating conditions. Maximum desired junction

temperature sets the maximum allowed internal power

dissipation as described below. In no case should the

maximum junction temperature be allowed to exceed

150

°C. Operating junction temperature (TJ) is given by TA

+ PD × qJA. The total internal power dissipation (PD) is the

sum of quiescent power (PDQ) and additional power

dissipation in the output stage (PDL) to deliver load power.

Quiescent power is the specified no-load supply current

times the total supply voltage across the part. PDL

(19)

(20)

(21)

OPA690 Datasheet(PDF) 23 Page - Texas Instruments

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