Datasheet

R
T
VCA810
-5V
E
O
V
C
I
BI
-
+5V
R
S
I
BN
E
NI
E
RS
4kTR
S
*
*
*
4kTR
T
VCA610
V
O
R
P
C
P
V
C
f =
-3dB
1
2 R Cp
P P
E = G E + (I R ) + 4kT(R + R )
O NI BI T S T
+ (I R )
BN S
(V/V)
·
2 2 2
E = E + (I R ) + 4kT(R + R )
N NI BI T S T
2 2
+ (I R )
BN S
2
VCA810
www.ti.com SBOS275F –JUNE 2003–REVISED DECEMBER 2010
GAIN CONTROL NOISE PERFORMANCE
The VCA810 gain is controlled by means of a The VCA810 offers 2.4nV/√Hz input-referred voltage
unipolar negative voltage applied between ground noise and 1.8 pA/√Hz input-referred current noise at
and the gain control input, pin 3. If use of the output a gain of +40dB. The input-referred voltage noise,
disable feature is required, a ground-referenced and the input-referred current noise terms, combine
bipolar voltage is needed. Output disable occurs for to give low output noise under a wide variety of
+0.15V ≤ V
C
≤ +2V, and produces greater than 80dB operating conditions. Figure 48 shows the op amp
of attenuation. The control voltage should be limited noise analysis model with all the noise terms
to +2V in disable mode, and –2.5V in gain mode in included. In this model, all noise terms are taken to
order to prevent saturation of internal circuitry. The be noise voltage or current density terms in either
VCA810 gain-control input has a –3dB bandwidth of nV/√Hz or pA/√Hz.
25MHz and varies with frequency, as shown in the
Typical Characteristics curves. This wide bandwidth,
although useful for many applications, can allow
high-frequency noise to modulate the gain control
input. In practice, this can be easily avoided by
filtering the control input, as shown in Figure 47. R
P
should be no greater than 100Ω so as not to
introduce gain errors by interacting with the gain
control input bias current of 6mA.
Figure 48. VCA810 Noise Analysis Model
The total output spot noise voltage can be computed
as the square root of the sum of all squared output
noise voltage contributors. Equation 8 shows the
general form for the output noise voltage using the
Figure 47. Control Line Filtering
terms shown in Figure 48.
(8)
GAIN CONTROL AND TEEPLE POINT
Dividing this expression by the gain will give the
When the VCA810 control voltage reaches −1.5V,
equivalent input-referred spot-noise voltage at the
also referred to as the Teeple point, the signal path
noninverting input as shown by Equation 9.
undergoes major changes. From 0V to the Teeple
point, the gain is controlled by one bank of amplifiers:
(9)
a low-gain VCA. As the Teeple point is passed, the
signal path is switched to a higher gain VCA. This
Evaluating these two equations for the VCA810 circuit
gain-stage switching can be seen most clearly in the
and component values shown in Figure 30
Noise Density vs Control Voltage Typical
(maximizing gain) will give a total output spot-noise
Characteristics curve (Figure 13). The output-referred
voltage of 272.3nV√Hz and a total equivalent
voltage noise density increases proportionally to the
input-referred spot-noise voltage of 2.72nV√Hz. This
control voltage and reaches a maximum value at the
total input-referred spot-noise voltage is higher than
Teeple point. As the gain increases and the internal
the 2.4nV√Hz specification for the VCA810 alone.
stages switch, the output-referred voltage noise
This reflects the noise added to the output by the
density drops suddenly and restarts its proportional
input current noise times the input resistance R
S
and
increase with the gain.
R
T
. Keeping input impedance low is required to
maintain low total equivalent input-referred spot-noise
voltage.
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