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ISL55210IRTZ-T7A Datasheet(PDF) 11 Page - Intersil Corporation |
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ISL55210IRTZ-T7A Datasheet(HTML) 11 Page - Intersil Corporation |
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11 / 18 page  ISL55210 11 FN7811.2 June 6, 2013 Most of the characterization curves start with Figure 28 then get different gains by changing the feedback resistor, RF, use different input transformers where then the RG is also adjusted to hold an input match, or vary the loading. For load tests below the 200Ω shown in Figure 28, a simple added shunt resistor is placed across the output pins. For loads >200Ω, the series and shunt load R's are adjusted to show that total load (including the 50Ω measurement load reflected through the 1:1 output measurement port transformer) and provide an apparent 50Ω differential source to that transformer. This output side transformer is for measurement purposes only and is not necessary for final applications circuits. There are output interface designs that do benefit from a transformer as part of the signal path, but the one shown at the right of Figure 28 is used only for characterization to get a doubly terminated 50Ω measurement path going differential to single ended. Where just the amplifier is tested, a 4 port network analyzer is used and the very simple test circuit of Figure 29 is implemented. This is used to extract the differential S21 curves and differential output impedance vs gain. Changing the gain is a simple matter of adjusting the two RF resistors of Figure 29. This circuit depends on the two AC coupled source 50Ω of the 4 port network analyzer and presents an AC coupled differential 100Ω load to the amplifier as the input impedance of the remaining two ports of the network analyzer. Using this measurement allows the full small single bandwidth of the ISL55210 to be exposed. Many of the other measurements are using I/O transformers that are limiting the apparent bandwidth to reduced level. Figure 16 shows a series of normalized differential S21 curves for gains of 12dB to 30dB in 6dB steps. These are simply stepping two feedback resistor values (RF) up from 200Ω to 1600Ω in 2X steps. The lowest gain of 12dB (4V/V) is showing a 2.2GHz small signal bandwidth. This response gets some bandwidth extension due to phase margin <60degree effects, but by the gain of 24dB (16V/V), the bandwidth is following a Gain Bandwidth type characteristic showing 300MHz bandwidth or >4GHz Gain Bandwidth Product (GBP). The closed loop differential output impedance of Figure 17 is simulated using Figure 29 in ADS. This shows a relatively low output impedance (<1Ω through 100MHz) constant with signal gain setting. Typical FDA outputs show a closed loop output impedance that increases with signal gain setting. The ISL55210 holds a more constant response vs gain due to internal design elements unique to this device. Common mode output measurements are made using the circuit Figure 30. Here, the outputs are summed together through two 100Ω resistors (still a 200Ω differential load) to a center point where the average, or common mode, output voltage may be sensed. This is coupled through a 1µF DC blocking capacitor and measured using 50Ω test equipment. The common mode source impedance for this circuit is the parallel combination of the 2Ω -100Ω elements, or 50Ω. Figure 18 uses this circuit to measure the small and large signal response from the VCM control pin to the output common mode. This pin includes an internal 50pF capacitor on the default bias network (to filter supply noise when there is no connection to this pin) which bandlimits the response to approximately 30MHz. This is far lower than the actual bandwidth of the common mode loop. Figure 19 uses this output CM measurement circuit with a large signal (2VP-P) differential output voltage (generated through the Vi path of Figure 30) to measure the differential to common mode conversion. Single Supply, Input Transformer Coupled, Design Considerations The characterization circuit of Figure 28 shows one possible input stage interface that offers several advantages. The ISL55210 can also support a DC coupled differential to differential or single ended input to differential requirement if needed. Where AC coupling is adequate, the circuit of Figure 28 simplifies the input common mode voltage control. If the source coming into this stage is single ended, the input transformer provides a zero power conversion to differential. The two gain resistors (RG in Figure 28) provide both the input termination impedance and the gain element for the amplifier. For minimum noise, only RG should be used and set to achieve the desired input impedance. Since the ISL55210 is a VFA device, these resistor values can be scaled up and down a bit more freely than a current feedback based FDA. FIGURE 29. TEST CIRCUIT #2 4-PORT S-PARAMETER MEASUREMENTS ISL55210 +3.3V + - VCM RF RF 1/2 OF A 4-port S-PARAMETER PD 50 10k 50 50 50 1/2 OF A 4-PORT S-PARAMETER FIGURE 30. TEST CIRCUIT #3 COMMON MODE AC OUTPUT MEASUREMENTS 1µF 1:1.4 ISL55210 +3.3V + - VCM ADT2-1T 1µF Vi PD OUTPUT VCM VCM INPUT 100 100 10k 50 50 50 50 50 50 200 200 |
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