VCA810_16 Datasheet, PDF (19/40 Pages) Texas Instruments – VCA810 High Gain Adjust Range, Wideband and Variable Gain Amplifier

www.ti.com
VCA810
SBOS275G – JUNE 2003 – REVISED DECEMBER 2015
8.3.5 Gain Control
The VCA810 gain is controlled by means of a unipolar negative voltage applied between ground and the gain
control input, pin 3. If use of the output disable feature is required, a ground-referenced bipolar voltage is
needed. Output disable occurs for 0.15 V ≤ V_C ≤ 2 V, and produces greater than 80 dB of attenuation. The
control voltage should be limited to 2 V in disable mode, and –2.5 V in gain mode to prevent saturation of internal
circuitry. The VCA810 gain-control input has a –3-dB bandwidth of 25 MHz and varies with frequency, as shown
in Typical Characteristics. 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 31. R_P should be no greater than 100 Ω so as not to introduce gain errors by interacting with the
gain control input bias current of 6 μA.
VCA610
C_P
R_P
V_O
1
f_-3dB = _2pRPCP
V_C
Figure 31. Control Line Filtering
8.3.6 Gain Control and Teeple Point
When the VCA810 control voltage reaches −1.5 V, also referred to as the Teeple point, the signal path
undergoes major changes. From 0 V to the Teeple point, the gain is controlled by one bank of amplifiers: a low-
gain VCA. As the Teeple point is passed, the signal path is switched to a higher gain VCA. This gain-stage
switching can be seen most clearly in Figure 13. The output-referred voltage noise density increases
proportionally to the control voltage and reaches a maximum value at the Teeple point. As the gain increases
and the internal stages switch, the output-referred voltage noise density drops suddenly and restarts its
proportional increase with the gain.
8.3.7 Noise Performance
The VCA810 offers 2.4-nV/√Hz input-referred voltage noise and 1.8-pA/√Hz input-referred current noise at a gain
of 40 dB. The input-referred voltage noise, and the input-referred current noise terms, combine to give low output
noise under a wide variety of operating conditions. Figure 32 shows the operational amplifier noise analysis
model with all the noise terms included. In this model, all noise terms are taken to be noise voltage or current
density terms in either nV/√Hz or pA/√Hz.
+5V
I_BN
R_S
* ^ENI
VCA810
E_O
E_{RS *}
4kTR_S
I_BI
V_C
-
R_T
-5V
* ^4kTRT
Figure 32. VCA810 Noise Analysis Model
Copyright © 2003–2015, Texas Instruments Incorporated
Product Folder Links: VCA810
Submit Documentation Feedback
19