Datasheet

DIFFERENTIAL NOISE PERFORMANCE
E =
O
e +(i R ) +4kTR
N N S S
´
2 2
4kTR
F
G
D
i
I
R
F
G
D
2
+22 ´ +2
DC ACCURACY AND OFFSET CONTROL
R
G
R
F
R
S
E
2
O
E
RS
E
N
I
N
I
N
R
F
R
S
E
RS
E
N
I
N
I
N
4kTR
G
4kTR
F
4kTR
S
4kTR
S
4kTR
F
G =1+
D
2 R´
F
R
G
(16)
E =
O
e +(i R ) +4kTR
N N S S
´
2 2
2 G´
D
´
2
+2(i R ) +2(4kTR G )
I F F
D
2
THS6214
www.ti.com ....................................................................................................................................................................................................... SBOS431 – MAY 2009
Dividing this expression by the differential noise gain
(G
D
= (1 + 2R
F
/R
G
)) 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 in such a 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 to 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 op amps, they do 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
V
OFF
= ± (NG × V
OS(MAX)
) + (I
BN
× R
S
/2 × NG)
± (I
BI
× R
F
)
As a reminder, the differential gain is expressed as:
where NG = noninverting signal gain
= ± (10 × 5mV) + (3.5 µ A × 25 Ω × 10)
± (1.24k Ω × 45 µ A)
The output noise can be expressed as shown below:
= ± 50mV + 0.875mV ± 55.8mV
V
OFF
= – 104.92mV to +106.67mV
(17)
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