When designing the amplifier/filter driving circuit, the ADC input full-scale voltage needs to be taken into

consideration. For example, the ADC3908D125 input full-scale is 1.9 VPP. When factoring in approximately

1dB for insertion loss of the filter, then the amplifier needs to deliver close to 2.1 VPP. The amplifier distortion

performance degrades with a larger output swing and considering the ADC common mode input voltage the

amplifier may not be able to deliver the full swing. The deviceADC3908D125 provides an output common mode

voltage of 1.25V and the THS4541 for example can only swing within 250mV of its negative supply. A unipolar

3.3V amplifier power supply limits the maximum voltage swing to approximately 2.8VPP. Hence, if a larger output

swing is required (factoring in filter insertion loss) then a negative supply for the amplifier is needed to eliminate

that limitation. Additionally, input voltage protection diodes may be needed to protect the ADC from over-voltage

events.

Table 8-2. Output voltage swing of THS4541 vs power supply

DEVICE

MIN OUTPUT VOLTAGE

MAX SWING WITH 3.3V/ 0V SUPPLY

THS4541

VS- + 250mV

2.8VPP

8.1.2 Detailed Design Procedure

8.1.2.1 Input Signal Path

The THS4541 provides a good low power option to drive the ADC inputs. Table 8-3 provides an overview of the

THS4541 with power consumption and usable frequency.

Table 8-3. Fully Differential Amplifier Options

DEVICE

CURRENT (IQ) PER CHANNEL

USABLE FREQUENCY RANGE

THS4541

10mA

< 70MHz

The low pass filter design (topology, filter order) is driven by the application. However, when designing the low

pass filter, the optimum load impedance for the amplifier must also be taken into consideration.

8.1.2.2 Sampling Clock

Applications operating with low input frequencies (such as DC to 20MHz) typically are less sensitive to

performance degradation due to clock jitter. The internal ADC aperture jitter improves with faster rise and fall

times (that is, square wave vs sine wave).

Termination of the clock input needs to be considered for long clock traces.

ADC3908D025, ADC3908D065, ADC3908D125

ADC3908S025, ADC3908S065, ADC3908S125

SBASAU9 – OCTOBER 2024

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