THS6204IPWPR datasheet(29/43 Pages) TI

29 / 43 page

DIFFERENTIAL NOISE PERFORMANCE

+(i

R ) +4kTR

4kTR

2 ´

DC ACCURACY AND OFFSET CONTROL

Driver

Ö4kTRS

Ö4kTRF

Ö4kTRS

Ö4kTRG

=1+

´ F

(16)

+(i

R ) +4kTR

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

THS6204

www.ti.com ................................................................................................................................................... SBOS416C – OCTOBER 2007 – REVISED APRIL 2009

Dividing this expression by the differential noise gain

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

As the THS6204 is used as a differential driver in

referred spot noise voltage at the noninverting input,

xDSL applications, it is important to analyze the noise

as shown in Equation 18.

in such a configuration. Figure 87 shows the op amp

noise model for the differential configuration.

(18)

Evaluating these equations for the THS6204 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 due to the

noninverting

input

bias

current

noise,

recommended

keep

the

noninverting

source

impedance as low as possible.

A current-feedback op amp such as the THS6204

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

worst-case +25°C input offset voltage and the two

Figure 87. Differential Op Amp Noise Analysis

input bias currents, gives a worst-case output offset

Model

range equal to:

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

As a reminder, the differential gain is expressed as:

RF)

where NG = noninverting signal gain

= ± (10 × 5mV) + (3

µA × 25Ω × 10) ± (1.24kΩ ×

The output noise can be expressed as shown below:

µA)

= ±50mV + 0.75mV ± 49.6mV

(17)

VOFF = –98.85mV to +100.35mV

Submit Documentation Feedback

Product Folder Link(s): THS6204

THS6204IPWPR Datasheet(PDF) 29 Page - Texas Instruments

THS6204IPWPR Datasheet(HTML) 29 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43

Datasheet Download

Link URL