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AD603-EB Datasheet(PDF) 6 Page - Analog Devices |
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AD603-EB Datasheet(HTML) 6 Page - Analog Devices |
6 / 14 page REV. C –6– AD603 Optionally, when a resistor is placed from FDBK to COMM, higher gains can be achieved. This fourth mode is of limited value because of the low bandwidth and the elevated output off- sets; it is thus not included in Figure 2. The gain of this amplifier in the first two modes is set by the ratio of on-chip laser-trimmed resistors. While the ratio of these resistors is very accurate, the absolute value of these resistors can vary by as much as ±20%. Thus, when an external resistor is connected in parallel with the nominal 6.44 k Ω ± 20% inter- nal resistor, the overall gain accuracy is somewhat poorer. The worst-case error occurs at about 2 k Ω (see Figure 4). REXT 1.2 10 1M 100 1k 10k 100k 1.0 0.8 0.6 0.4 0.2 0.0 –0.2 –0.4 –0.6 –0.8 –1.0 –1:VdB (OUT) – (–1):VdB (OREF) VdB (OUT) – VdB (OREF) Figure 4. Worst-Case Gain Error, Assuming Internal Resis- tors Have a Maximum Tolerance of –20% (Top Curve) or +20% (Bottom Curve) While the gain-bandwidth product of the fixed-gain amplifier is about 4 GHz, the actual bandwidth is not exactly related to the maximum gain. This is because there is a slight enhancing of the ac response magnitude on the maximum bandwidth range, due to higher order poles in the open-loop gain function; this mild peaking is not present on the higher gain ranges. Figure 2 shows how optional capacitors may be added to extend the frequency response in high gain modes. CASCADING TWO AD603S Two or more AD603s can be connected in series to achieve higher gain. Invariably, ac coupling must be used to prevent the dc offset voltage at the output of each amplifier from overload- ing the following amplifier at maximum gain. The required high pass coupling network will usually be just a capacitor, chosen to set the desired corner frequency in conjunction with the well- defined 100 Ω input resistance of the following amplifier. For two AD603s, the total gain-control range becomes 84 dB (two times 42.14 dB); the overall –3 dB bandwidth of cascaded stages will be somewhat reduced. Depending on the pin-strapping, the gain and bandwidth for two cascaded amplifiers can range from –22 dB to +62 dB (with a bandwidth of about 70 MHz) to +22 dB to +102 dB (with a bandwidth of about 6 MHz). There are several ways of connecting the gain-control inputs in cascaded operation. The choice depends on whether it is impor- tant to achieve the highest possible Instantaneous Signal-to-Noise Ratio (ISNR), or, alternatively, to minimize the ripple in the gain error. The following examples feature the AD603 programmed for maximum bandwidth; the explanations apply to other gain/ bandwidth combinations with appropriate changes to the arrange- ments for setting the maximum gain. Sequential Mode (Optimal S/N Ratio) In the sequential mode of operation, the ISNR is maintained at its highest level for as much of the gain control range possible. Figure 5 shows the SNR over a gain range of –22 dB to +62 dB, assuming an output of 1 V rms and a 1 MHz bandwidth; Figure 6 shows the general connections to accomplish this. Here, both the positive gain-control inputs (GPOS) are driven in parallel by a positive-only, ground-referenced source with a range of 0 V to +2 V, while the negative gain-control inputs (GNEG) arc biased by stable voltages to provide the needed gain-offsets. These volt- ages may be provided by resistive dividers operating from a common voltage reference. VC 90 –0.2 2.2 0.2 0.6 1.0 1.4 1.8 85 80 75 70 65 60 55 50 Figure 5. SNR vs. Control Voltage—Sequential Control (1 MHz Bandwidth) The gains are offset (Figure 7) such that A2’s gain is increased only after A1’s gain has reached its maximum value. Note that for a differential input of –600 mV or less, the gain of a single amplifier (A1 or A2) will be at its minimum value of –11.07 dB; for a differential input of +600 mV or more, the gain will be at its maximum value of 31.07 dB. Control inputs beyond these limits will not affect the gain and can be tolerated without dam- age or foldover in the response. This is an important aspect of the AD603’s gain-control response. (See the Specifications sec- tion of this data sheet for more details on the allowable voltage range) The gain is now Gain (dB) = 40 VG + GO (3) where VG is the applied control voltage and GO is determined by the gain range chosen. In the explanatory notes that follow, we assume the maximum-bandwidth connections are used, for which GO is –20 dB. |
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