PSI DRS4 - Analog Input Signal Connection

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DRS4 Evaluation Board User’s Manual

Page 28 of 43

power supply AV

powers only the domino circuit of the DRS4 chip and directly influences

the jitter of the sampling frequency. Long term variations in this power supply (seconds…)

are regulated by the on-chip PLL, but high frequency noise in the MHz region leads directly

to an increase of the PLL jitter. Therefore the evaluation board contains two separate 2.5V

linear regulators for the DRS4 chip, one for the AV

power and one for the DV

power.

Although the DV

power is called “digital power”, it powers also the analog output buffers

of the DRS4 chip and needs the same good quality than the AV

power in order to minimize

the noise of the board.

The DRS4 chip also contains two grounds AGND and DGND. They can be either routed

separately on the board and be connected at the power source, or they can be directly

connected to an overall dedicated ground plane of the PCB. Tests have been shown that the

latter choice gives slightly less noise.

The bottom of the QFN76 package of the DRS4 has an exposed paddle connected to the

internal DGND. It is recommended that this paddle is matched by a PCB pad of similar size

connected to analog ground to achieve the best electrical and thermal performance of the

DRS4. The copper plane should have several vias to achieve a good heat dissipation to flow

through the PCB as shown in Figure 3:

Figure 3: PCB pad under the DRS4 chip

These vias should be solder-filled or plugged. The maximum power dissipation of the DRS4

chip is not critical (350 mW), but an improved thermal stability helps the performance of any

analog chip. To maximize the coverage and adhesion between the DRS4 and the PCB, the

copper plane could be partitioned into several uniform sections, providing several tie points

during the reflow process.

4.2. Analog Input

If non-differential signals should be digitized with the DRS4 chip, they must be converted

into differential signals for the DRS4 inputs. The simplest solution is to connect the IN- inputs

to AGND and to connect the signals directly to the IN+ inputs. This method has however the

disadvantage that the crosstalk and noise immunity of the DRS4 chip are worsened. The

evaluation board V3 uses differential drivers THS4508 from Texas Instruments

for this

purpose. These drivers were carefully selected since they have to drive the relatively high

DRS4 input current of almost 1 mA and capacitive load without compromising the analog

bandwidth. The current design gives about 700 MHz (-3 dB) with moderate power

consumption, so that the evaluation board can still be powered from the USB power (500 mA