Microwave Engineering Europe - March 2009 - (Page 22) 22 WIRELESS INFRASTRUCTURE glitches generated when the input is switched from hold mode to sample mode, during which the hold capacitor sees a sudden change in voltage. The filter output amplifier can source or sink more than 20 mA load current and is short-circuit protected at ±65 mA current limiting. This combines with the device’s high amplifier gain-bandwidth helps to provide low impedance drive to absorb the current glitches inherent in many high speed ADCs with minimum disturbance. Moreover, the LTC6601-1 output stage is a rail-to-rail design that can drive ±1 Vp-p differential into an external 50 • load. Figure 1 also shows the filter driving differentially into an ADC. It is a good idea to add a RC network to decouple the LTC6601-1 filter outputs from the ADC inputs. The shunt capacitors provide the bulk of the transient charge during ADC sampling, while the series resistors provide attenuation and decoupling of the transient noise. The value of the R1 and C1 depend on the bandwidth of the signal and the desired noise band-limiting. A good starting point is a 25 • series resistance. Too high a resistance could unnecessarily lengthen settling time and increase attenuation through the dynamic input resistance of the ADC. Too little resistance may not provide sufficient damping. Then the RC filter pole should be chosen to be higher than the LTC6601-1 filter corner so that it does not significantly alter the phase shift and attenuation to the desired filter response. At the same time, the RC pole should be low enough to provide some broadband noise roll-off. A good rule of thumb is to set the RC pole 3 to 4 times higher than the desired filter corner. In Figure 1, the LTC6601-1 is DC coupled to the ADC. The DC common-mode voltage must satisfy both the filter and the ADC. The LTC6601-1’s output common-mode rejection ratio is fairly robust at 70 dB. While the ADC’s input common-mode range is more limited and that its conversion performance depends on a relatively narrow common-mode voltage range. So the ADC’s input common-mode of 1.250 V pin is used to drive the LTC6601-1’s output common-mode pin (pin 12) which has relatively high impedance. On the input side, the LTC6601-1 has a balanced differential arrangement. But it can be driven from a single-ended source if desired. To do so, one side of the differential input should be AC-grounded as shown in Figure 1. Then the other differential input can be driven with a single-ended source. Microwave Engineering Europe ● March 2009 ● Dual broadband filters have matched performance to 15 MHz The LT6604 family of dual broadband filters is designed to provide tight matching of phase and amplitude performance over its rated frequency bandwidth. The family is available in fixed 2.5 MHz, 5 MHz, 10 MHz and 15 MHz cutoff bandwidths. Their responses have a 4th order approximate Chebyshev roll-off skirt while maintaining a tight 0.6 dB gain flatness in the passband up to the specified cutoff frequencies. They are designed for today’s new generation of advanced communications receivers, instrumentation, and signal processing equipment requiring exacting multichannel filtering and matching characteristics. Configured as single supply (3 V and 5 V) differential input and output filters, the LT6604 family can also operate single- ended inputs, and can be powered from ±5 V supplies. The designers can set their filter gain using two external resistors without altering the filters’ frequency response. The LT6604 filters have an internally biased output DC common-mode voltage that is onehalf of the supply voltage which automatically biases the part regardless of the supply voltage used. But when driving into an A/D converter that has an input bias voltage other than the mid-supply, it is best to override the filter’s internal common-mode voltage with the ADC’s so the interface is seamless – as long as the driving source impedance is low (<100 • ). Higher order software programmable filters offer system flexibility LTC6602 and LTC6603 are dual, matched filters with software programmability which enhances system flexibility. Both filters are Figure 2: — a) The left-hand plot shows the LTC6602 frequency response that can be controlled by the SPI bus: — b) The right-hand plot shows LTC6603 frequency response steps using the SPI interface. Table 1: LTC6602 cutoff frequency control, RBIAS = 54.9 k, fCLK = 90 MHz. Table 2: LTC6603 cutoff frequency control, RBIAS = 30.9 k, fCLK = 80 MHz. www.mwee.com http://www.mwee.com
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