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AD605AR-REEL7 Folha de dados(PDF) 10 Page - Analog Devices |
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AD605AR-REEL7 Folha de dados(HTML) 10 Page - Analog Devices |
10 / 12 page AD605 –10– REV. C The AFA makes a differential input structure possible since one of its inputs (G1) is fully differential; this input is made up of a distributed gm stage. The second input (G2) is used for feed- back. The output of G1 will be some function of the voltages sensed on the attenuator taps which is applied to a high-gain amplifier (A0). Because of negative feedback, the differential input to the high-gain amplifier has to be zero; this in turn implies that the differential input voltage to G2 times gm2 (the transcon- ductance of G2) has to be equal to the differential input voltage to G1 times gm1 (the transconductance of G1). Therefore the overall gain function of the AFA is V V gm gm RR R OUT ATTEN =× × 1 2 12 2 (7) where VOUT is the output voltage, VATTEN is the effective voltage sensed on the attenuator, (R1 + R2)/R2 = 42, and gm1/gm2 = 1.25; the overall gain is thus 52.5 (34.4 dB). The AFA has additional features: (1) inverting the output signal by switching the positive and negative input to the ladder network; (2) the possibility of using the –IN input as a second signal input; and (3) independent control of the DSX common-mode voltage. Under normal operating conditions it is best to just connect a decoupling capacitor to pin VOCM in which case the common- mode voltage of the DSX is half the supply voltage; this allows for maximum signal swing. Nevertheless, the common-mode voltage can be shifted up or down by directly applying a voltage to VOCM. It can also be used as another signal input, the only limitation being the rather low slew rate of the VOCM buffer. If the dc level of the output signal is not critical, another coupling capacitor is normally used at the output of the DSX; again this is done for level shifting and to eliminate any dc offsets contrib- uted by the DSX (see AC Coupling section). The gain range of the DSX is programmable by a resistor con- nected between pins FBK and OUT. The possible ranges are –14 dB to +34.4 dB when the pins are shorted together, to 0 dB to +48.4 dB when FBK is left open. Note that for the higher gain range, the bandwidth of the amplifier is reduced by a factor of five to about 8 MHz since the gain increased by 14 dB. This is the case for any constant gain bandwidth product amplifier which includes the active feedback amplifier. APPLICATIONS The basic circuit in Figure 4 shows the connections for one chan- nel of the AD605 with a gain range of –14 dB to +34.4 dB. The signal is applied at Pin 3. The ac-coupling capacitors before pins –IN1 and +IN1 should be selected according to the required lower cutoff frequency. In this example, the 0.1 µF capacitors to- gether with the 175 Ω of each of the DSX input pins provides a –3 dB high-pass corner of about 9.1 kHz. The upper cutoff frequency is determined by the amplifier and is 40 MHz. 14 13 12 11 16 15 10 9 8 1 2 3 4 7 6 5 VREF GND1 +IN1 –IN1 VGN1 OUT1 FBK1 VPOS –IN2 +IN2 GND2 VPOS FBK2 OUT2 VOCM VGN2 AD605 0.1 F 0.1 F VGN VIN 0.1 F 5V 0.1 F OUT 2.500V Figure 4. Basic Connections for a Single Channel As shown here, the output is ac-coupled for optimum perfor- mance. In the case of connecting to the 10-bit 40 MSPS A/D converter AD9050, ac coupling can be eliminated as long as pin VOCM is biased by the same 3.3 V common-mode voltage as the AD9050. Pin VREF requires a voltage of 1.25 V to 2.5 V, with gain scaling between 40 dB/V and 20 dB/V, respectively. Voltage VGN controls the gain; its nominal operating range is from 0.25 V to 2.65 V for 20 dB/V gain scaling, and 0.125 V to 1.325 V for 40 dB/V scaling. When this pin is taken to ground, the channel will power down and disable its output. Connecting Two Amplifiers to Double the Gain Range Figure 5 shows the two channels of the AD605 connected in series to provide a total gain range of 96.8 dB. When R1 and R2 are shorts, the gain range will be from –28 dB to +68.8 dB with a slightly reduced bandwidth of about 30 MHz. The reduction in bandwidth is due to two identical low-pass circuits being connected in series; in the case of two identical single-pole low- pass filters, the bandwidth would be reduced by exactly √2. If R1 and R2 are replaced by open circuits, i.e., Pins FBK1 and FBK2 are left unconnected, then the gain range will shift up by 28 dB to 0 dB to +96.8 dB. As noted earlier, the bandwidth of each individual channel will be reduced by a factor of 5 to about 8 MHz since the gain increased by 14 dB. In addition, there is still the √2 reduction because of the series connection of the two channels which results in a final bandwidth of the higher gain version of about 6 MHz. 14 13 12 11 16 15 10 9 8 1 2 3 4 7 6 5 VREF GND1 +IN1 –IN1 VGN1 OUT1 FBK1 VPOS –IN2 +IN2 GND2 VPOS FBK2 OUT2 VOCM VGN2 AD605 C2 0.1 F VGN VIN R1 5V OUT 2.500V C1 0.1 F C3 0.1 F C4 0.1 F C6 0.1 F R2 C5 0.1 F Figure 5. Doubling the Gain Range with Two Amplifiers |
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