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AD8039AR-REEL7 Datenblatt(PDF) 10 Page - Analog Devices |
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AD8039AR-REEL7 Datenblatt(HTML) 10 Page - Analog Devices |
10 / 12 page REV. B –10– AD8038/AD8039 The AD9203 works best when the common-mode voltage at the input is at the midsupply or 2.5 V. The output stage design of the AD8039 makes it ideal for driving these types of ADCs. In this circuit, one of the op amps is configured in the inverting mode, while the other is in the noninverting mode. However, to provide better bandwidth matching, each op amp is configured for a noise gain of +2. The inverting op amp is configured for a gain of –1, while the noninverting op amp is configured for a gain of +2. Each has a very similar ac response. The input signal to the noninverting op amp is divided by 2 to normalize its voltage level and make it equal to the inverting output. The outputs of the op amps are centered at 2.5 V, which is the midsupply level of the ADC. This is accomplished by first taking the 2.5 V reference output of the ADC and dividing it by 2 with a pair of 1 k Ω resistors. The resulting 1.25 V is applied to each op amp’s positive input. This voltage is then multiplied by the gain of the op amps to provide a 2.5 V level at each output. Low Power Active Video Filter Some composite video signals derived from a digital source contain clock feedthrough that can limit picture quality. Active filters made from op amps can be used in this application, but they will consume 25 mW to 30 mW for each channel. In power-sensitive applications, this can be too much, requiring the use of passive filters that can create impedance matching prob- lems when driving any significant load. The AD8038 can be used to make an effective low-pass active filter that consumes one-fifth of the power consumed by an active filter made from an op amp. Figure 4 shows a circuit that uses an AD8038 to create a single ±2.5 V supply, three-pole Sallen-Key filter. This circuit uses a single RC pole in front of a standard two-pole active section. 0.1 F +2.5V 10 F –2.5V 0.1 F 10 F C3 33pF R3 49.9k RF 1k 680pF R5 49.9k R2 499k C1 100pF R1 200k R4 49.9k AD8038 VIN VOUT Figure 4. Low-Pass Filter for Video Figure 5 shows the frequency response of this filter. The response is down 3 dB at 6 MHz, so it passes the video band with little attenuation. The rejection at 27 MHz is 45 dB, which provides more than a factor of 100 in suppression of the clock components at this frequency. FREQUENCY – MHz 0.1 110 100 –10 10 0 –20 –30 –40 –50 –60 Figure 5. Video Filter Response |
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Ähnliche Beschreibung - AD8039AR-REEL7 |
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