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AD8330 Datenblatt(PDF) 18 Page - Analog Devices |
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AD8330 Datenblatt(HTML) 18 Page - Analog Devices |
18 / 32 page AD8330 Data Sheet Rev. H | Page 18 of 32 VIN VMAG TIME (ns) 0.10 –400 –300 –200 –100 0 100 200 300 0.05 0 –0.05 –0.10 1.2 1.0 0.8 0.6 0.4 0.2 0 2.5 2.0 1.5 1.0 0.5 0 –0.5 –1.0 –1.5 –2.0 VOUT Figure 50. Using VMAG in Modulation Mode Another gain related feature allows both gain control ranges to be accurately raised by 200 mV. To enable this offset, open circuit CMGN (Pin 6, LFCSP; Pin 8, QSOP) and add a 0.1 µF capacitor to ground. In use, the nominal range for VDBS extends from 0.2 V to 1.7 V and VMAG from 0.2 V to 5.2 V. These specifications apply for any supply voltage. This allows the use of DACs whose output range does not include ground as sources for the gain control function(s). Note that the 200 mV that appears on this pin affects the response to an externally applied VMAG, but when Pin VMAG is unconnected, the internally set default value of 0.5 V still applies. Furthermore, Pin CMGN can, if desired, be driven by a user- supplied voltage to reposition the baseline for VDBS (or for an externally applied VMAG) to any other voltage up to 500 mV. In all cases, the gain scaling, its law conformance, and temperature stability are unaffected. Two Classes of Variable Gain Amplifiers Note that there are two broad classes of VGAs. The first type is designed to cope with a very wide range of input amplitudes and, by virtue of its gain control function, compress this range down to an essentially constant output. This is the function needed in an AGC system. Such a VGA is called an IVGA, referring to a structure optimized to address a wide range of input amplitudes. By contrast, an OVGA is optimized to deliver a wide range of output values while operating with an essentially constant input amplitude. This function might be needed, for example, in providing a variable drive to a power amplifier. It is apparent from the foregoing sections that the AD8330 is both an IVGA and an OVGA in one package. This is an unusual and possibly confusing degree of versatility for a VGA; therefore, these two distinct control functions are described at separate points throughout this data sheet to explain the operation and applications of this product. It is, nevertheless, useful to briefly describe the capabilities of these features when used together. Amplitude/Phase Response The ac response of the AD8330 is remarkably consistent not only over the full 50 dB of its basic gain range, but also with changes of gain due to alteration of VMAG, as demonstrated in Figure 51. This is an overlay of two sets of results: first, with a very low VMAG of 16 mV that reduces the overall gain by 30 dB [20 × log10(500 mV/16 mV)]; second, with VMAG = 5 V that increases the gain by 20 dB = 20 × log10(5 V/0.5 V). FREQUENCY (Hz) 90 100k 10k 30 –10 –350 50 10 –30 1M 10M 100M 300M 70 –50 –50 –100 –150 –200 –250 –300 0 G = +70dB G = –20dB 100k 1M 10M 100M 300M Figure 51. AC Performance over a 100 dB Gain Range Obtained by Using Two Values of VMAG This 50 dB step change in gain produces two sets of gain curves, having a total gain span of 100 dB. It is apparent that the ampli- tude and phase response are essentially independent of the gain over this wide range, an aspect of the AD8330 performance potential unprecedented in any prior VGA. It is unusual for an application to require such a wide range of gains; and, as a practical matter, the peak output voltage for VMAG = 16 mV is reduced by the factor 16/500, compared to its nominal value of ±2 V, to only ±64 mV. As previously noted, most applications of VGAs require that they operate in a mode that is predominantly of either an IVGA or OVGA style, rather than mixed modes. With this limitation in mind, and simply to illustrate the unusual possibilities afforded by the AD8330, note that, with appropriate drive to VDBS and VMAG in tandem, the gain span is a remarkable 120 dB, extending from −50 dB to +70 dB, as shown in Figure 52 for operation at 1 MHz and 100 MHz. In this case, VDBS and VMAG are driven from a common control voltage, VGAIN, that varies from 1.2 mV to 5 V, with 30% (1.5/5) of VGAIN applied to VDBS, and 100% applied to VMAG. The gain varies in a linear-in-dB manner with VDBS, although the response from VMAG is linear-in-magnitude. Consequently, the overall numerical gain as the product of these two functions is V 6 . 0 10 3 . 0 V 5 . 0 / GAIN V GAIN V GAIN × × = (7) In rare cases where such a wide gain range is of value, the calibration is still accurate and the temperature is stable. |
Ähnliche Teilenummer - AD8330_16 |
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Ähnliche Beschreibung - AD8330_16 |
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