SBOS778D April 2016 – April 2021 THS4551
Driving a 2-MHz ±0.2-V square wave into this circuit (using a TINA-TI™ simulation file for the circuit of Figure 10-17) gives the response shown in Figure 10-18 at the ADC. The red trace is a –1-dBFS, 1.8-VPP square wave at the ADC input pins. The gray trace is the input signal at the RT 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.
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 VCM output. Consider this effect carefully when using higher resistor values in the interface at the ADC.