THS4551 Datasheet, PDF (53/70 Pages) Texas Instruments – Low-Noise, Precision, 150-MHz, Fully Differential Amplifier

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THS4551
SBOS778A – APRIL 2016 – REVISED AUGUST 2016
10.2.3.3 Application Curve
Driving a 2-MHz ±0.2-V square wave into this circuit gives the response shown in Figure 94 at the ADC. The red
trace is a –1-dBFS, 1.8-V_PP square wave at the ADC input pins. The gray trace is the input signal at the R_T
termination resistor. The black trace is the common-mode voltage at the FDA input pins. Note that the input pin
voltage swing stays above ground and in range for this bipolar input, single, 3.3-V supply design.
1.5
1
0.5
0
-0.5
-1
-1.5
0
VIN+
V_OUT
V_IN
100 200 300 400 500 600 700 800 900 1000
Time (ns)
D076
Figure 94. Time-Domain Waveform
Unbuffered pipeline ADCs draw a clock-rate-dependent input common-mode current. For the ADC3241, this
input current is specified as 1.5 µA per MSPS. Operating at 25 MSPS, the common-mode current drops the
common-mode voltage from 0.95 V at the THS4551 outputs by 37.5 µA × 45.8 Ω = 1.7 mV to 0.9483 V. This
value is well within the allowed ±25-mV common-mode deviation from the ADC V_CM output. Consider this effect
carefully when using higher resistor values in the interface at the ADC.
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