In some cases, it may be beneficial to digitally control the signal level. An example of an upstream cable modem driver is the AD8325.
Cable modems are becoming more popular because data rates are much higher than standard dial-up connections. In addition to receiving data (downstream), cable modems can also transmit data (upstream). This requires the use of a low distortion digitally controlled variable gain amplifier that can drive a 75 coaxial cable at a nominal level of 1 V rms (+11.2 dBm or 60 dBmV). The AD8325 is a line of cable TV (CATV) uplink drivers for this application. The gain of the AD8325 is controlled by an 8-bit serial word that determines the desired gain in the 59.45 dB range, resulting in a gain variation of 0.7526 dB/LSB. The block diagram of the AD8325 is shown in Figure 8 below.
Figure 8: AD8325 CATV Digitally Controlled Variable Gain Amplifier
The AD8325 has a variable attenuator core that digitally controls attenuation from 0 dB to –59.45 dB. The input buffer has a gain of approximately + 30 dB, resulting in a total gain range of –29.45 dB to +30.0 dB. In power-up mode, the AD8325 includes four analog functions. The input amplifier (preamplifier) ​​can be configured in a single-ended or differential configuration. The 8-bit control word is decoded into a 3-bit word and a 9-bit word. The former drives the wiper stage (precision gain adjustment) and the latter drives the attenuation core (DAC). The 0.7526 dB/LSB resolution is achieved in the vernier stage with a total attenuation of approximately 5.25 dB. After the vernier level, the DAC provides a batch of AD8325 attenuation (9 bits or 54 dB). The signals in the preamplifier and vernier gain blocks are differential to improve PSRR and linearity. The differential current is fed from the DAC to the output stage, which amplifies these currents to the appropriate level required to drive the 75 load. The AD8325 maintains a constant 75-channel dynamic output impedance for both power-up and shutdown, a major performance and cost advantage of the device. The output stage uses negative feedback to achieve a 75 differential dynamic output impedance. This eliminates the need for an external 75-side termination resistor, which in turn produces twice the effective output voltage of a standard op amp.
These features allow the AD8325 to operate from a single +5 V supply and still provide the required output power. With a 21 MHz bandwidth and an output level of up to 1 V rms (+11.2 dBm), the distortion performance is -57 dBc.
The AD8370 is a low cost, digitally controlled, variable gain amplifier that provides precision gain control, high IP3 and low noise figure. The block diagram is shown in Figure 9.
Figure 9: 750MHz Digitally Controlled VGA AD8370
The AD8370 has excellent distortion performance and wide bandwidth. For wide input dynamic range applications, the AD8370 offers two input ranges: high gain mode and low gain mode. A vernier 7-bit transconductance (Gm) stage provides a 28 dB gain range with better than 2 dB resolution and a 22 dB gain range with better than 1 dB resolution. The second gain range is 17 dB higher than the first and can be selected to improve noise performance. The AD8370's power supply is provided by the logic level of the PWUP pin, which consumes less than 4 mA in shutdown mode, providing excellent input-output isolation. The gain setting remains the same when operating in shutdown mode.
The gain control of the AD8370 is implemented by an 8-bit serial gain control word. The MSB selects between two gain ranges, and the remaining 7 bits adjust the total gain in precise linear gain steps.
The AD8375 is a differential variable gain amplifier consisting of a 150 digitally controlled passive attenuator followed by a high linearity transconductance amplifier, as shown in Figure 10.
Figure 10: 630MHz Low Distortion Digitally Controlled VGA AD8375
A 5-bit binary code changes the attenuation setting in 1 dB steps, resulting in a device gain from 20 dB (code 0) to −4 dB (code 24 and above). At the maximum gain setting, the noise figure of the device is approximately 8 dB and increases with decreasing gain. The amount of increase in the noise figure is equal to the amount of decrease in gain. The linearity of the device measured at the output is first order and independent of the gain setting. With a gain between 0 dB and 20 dB, the 150-load OIP3 at 140 MHz is approximately 50 dBm (3 dBm per tone). When the gain is set below 0 dB, it drops to approximately 45 dBm.
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