VCA810_16 Datasheet, PDF (20/40 Pages) Texas Instruments – VCA810 High Gain Adjust Range, Wideband and Variable Gain Amplifier

VCA810
SBOS275G – JUNE 2003 – REVISED DECEMBER 2015
www.ti.com
The total output spot noise voltage can be computed as the square root of the sum of all squared output noise
voltage contributors. Equation 4 shows the general form for the output noise voltage using the terms shown in
Figure 32.
E_O = G · _(V/V) E_NI2 + (I_BIR_T)² + (I_BNR_S)² + 4kT(R_S + R_T)
(4)
Dividing this expression by the gain will give the equivalent input-referred spot-noise voltage at the noninverting
input as shown by Equation 5.
E_N = E_NI2 + (I_BIR_T)² + (I_BNR_S)² + 4kT(R_S + R_T)
(5)
Evaluating these two equations for the VCA810 circuit and component values shown in Figure 34 (maximizing
gain) will give a total output spot-noise voltage of 272.3 nV√Hz and a total equivalent input-referred spot-noise
voltage of 2.72 nV√Hz. This total input-referred spot-noise voltage is higher than the 2.4-nV√Hz specification for
the VCA810 alone. This reflects the noise added to the output by the input current noise times the input
resistance R_S and R_T. Keeping input impedance low is required to maintain low total equivalent input-referred
spot-noise voltage.
8.3.8 Input and ESD Protection
The VCA810 is built using a very high-speed complementary bipolar process. The internal junction breakdown
voltages are relatively low for these very small geometry devices. These breakdowns are reflected in Absolute
Maximum Ratings
All pins on the VCA810 are internally protected from ESD by means of a pair of back-to-back, reverse-biased
diodes to either power supply, as shown in Figure 33. These diodes begin to conduct when the pin voltage
exceeds either power supply by about 0.7 V. This situation can occur with loss of the amplifier power supplies
while a signal source is still present. The diodes can typically withstand a continuous current of 30 mA without
destruction. To ensure long-term reliability, however, diode current should be externally limited to 10 mA
whenever possible.
ESD Protection diodes internally
+V_S
connected to all pins.
External
Pin
Internal
Circuitry
-V_S
Figure 33. Internal ESD Protection
8.4 Device Functional Modes
The VCA824 functions as a differential input, single-ended output variable gain amplifier. This functional mode is
enabled by applying power to the amplifier supply pins and is disabled by turning the power off.
The gain is continuously variable through the analog gain control input. The gain is set by an external, analog,
control voltage as shown in the functional block diagram. The signal gain is equal to G = (V/V) 10–2(V + 1) as
detailed in Overview. The gain changes in a linear in dB fashion with over 80 dB of gain range from –2-V to –0-V
control voltage. As with most other differential input amplifiers, inputs can be applied to either one or both of the
amplifier inputs. The amplifier gain is controlled through the gain control pin.
In addition to gain control, the gain control pin can also be used to disable the amplifier. This is accomplished by
applying a slightly positive voltage to this pin. This is detailed Feature Description.
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