THS6214IPWPR datasheet(29/41 Pages) TI

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DIFFERENTIAL NOISE PERFORMANCE

E =

+(i

R ) +4kTR

4kTR

IRF

2 ´

DC ACCURACY AND OFFSET CONTROL

4kTR

=1+

(16)

E =

+(i

R ) +4kTR

+2(i R ) +2(4kTR G )

THS6214

www.ti.com ....................................................................................................................................................................................................... SBOS431 – MAY 2009

Dividing this expression by the differential noise gain

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

The THS6214 is designed to be used as a differential

referred spot noise voltage at the noninverting input,

driver in xDSL applications. Therefore, it is important

as shown in Equation 18.

to analyze the

noise

such

configuration.

Figure 87 shows the op amp noise model for the

differential configuration.

(18)

Evaluating these equations for the THS6214 ADSL

circuit and component values of Figure 82 gives a

total output spot noise voltage of 38.9nV/

√Hz and a

total equivalent input spot noise voltage of 7nV/

√Hz.

In order to minimize the output noise as a result of

the noninverting input bias current noise, it is

recommended

keep

the

noninverting

source

impedance as low as possible.

A current-feedback op amp such as the THS6214

provides exceptional bandwidth in high gains, 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. While bias

current cancellation techniques are very effective with

most

voltage-feedback

amps,

they

not

generally reduce the output dc offset for wideband

current-feedback op amps. Because the two input

bias currents are unrelated in both magnitude and

polarity, matching the input source impedance to

reduce error contribution to the output is ineffective.

Evaluating the configuration of Figure 81, using a

worst-case condition at +25°C input offset voltage

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

Figure 87. Differential Op Amp Noise Analysis

output offset range equal to:

Model

VOFF = ±(NG × VOS(MAX)) + (IBN × RS/2 × NG)

±(IBI × RF)

As a reminder, the differential gain is expressed as:

where NG = noninverting signal gain

±(10

5mV)

(3.5

µA × 25Ω × 10)

±(1.24k

Ω × 45

µA)

The output noise can be expressed as shown below:

= ±50mV + 0.875mV ± 55.8mV

VOFF = –104.92mV to +106.67mV

(17)

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