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VCA824IDG4 Datasheet(PDF) 21 Page - Texas Instruments

Part No. VCA824IDG4
Description  Ultra-Wideband, > 40-dB Gain Adjust Range, Linear in V/V Variable Gain Amplifier
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Maker  TI1 [Texas Instruments]
Homepage  http://www.ti.com
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VCA824IDG4 Datasheet(HTML) 21 Page - Texas Instruments

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External
Pin
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Internal
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VCA824
www.ti.com
SBOS394D – NOVEMBER 2007 – REVISED JANUARY 2016
Device Functional Modes (continued)
8.4.4 Output Voltage Dynamic Range
With its large output current capability and its wide output voltage swing of ±3.9 V typical on 100-
Ω load, it is
easy to forget other types of limitations that the VCA824 can encounter. For these limitations, careful analysis
must be done to avoid input stage limitation: either voltage or IRG current. Note that if control pin VG varies, the
gain limitation may affect other aspects of the circuit.
8.4.5 Bandwidth
The output stage of the VCA824 is a wideband current-feedback amplifier. As such, the feedback resistance is
the compensation of the last stage. Reducing the feedback element and maintaining the gain constant limits the
useful range of IRG, and therefore, reduces the gain adjust range. For a given gain, reducing the gain element
limits the maximum achievable output voltage swing.
8.4.6 Offset Adjustment
As a result of the internal architecture used on the VCA824, the output offset voltage originates from the output
stage and from the input stage and multiplier core. Figure 67 shows how to compensate both sources of the
output offset voltage. Use this procedure to compensate the output offset voltage: starting with the output stage
compensation, set VG = –1 V to eliminate all offset contribution of the input stage and multiplier core. Adjust the
output stage offset compensation potentiometer. Finally, set VG = 1 V to the maximum gain and adjust the input
stage and multiplier core potentiometer. This procedure effectively eliminates all offset contribution at the
maximum gain. Because adjusting the gain modifies the contribution of the input stage and the multiplier core,
some residual output offset voltage remains.
8.4.7 Noise
The VCA824 offers 6 nV/
√Hz input-referred voltage noise density at a gain of 10 V/V and 2.6-pA/√Hz input-
referred current noise density. The input-referred voltage noise density considers that all noise terms (except the
input current noise but including the thermal noise of both the feedback resistor and the gain resistor) are
expressed as one term.
This model is formulated in Equation 4 and Figure 68.
(4)
A more complete model is shown in Figure 69. For additional information on this model and the actual modeled
noise terms, please contact the High-Speed Product Application Support team at www.ti.com.
8.4.8 Input and ESD Protection
The VCA824 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 the
Absolute Maximum Ratings.
All pins on the VCA824 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 64. 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.
Figure 64. Internal ESD Protection
Copyright © 2007–2016, Texas Instruments Incorporated
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