Analog Interfaces - MLA Generation 2

OUT 1 and OUT 2

The fast (up to 250 MSamples/sec) output ports have a DA-converter, a low pass filter, and an operational amplifier. The DA-converter is a single chip with two channels, where both channels are differential current output. The low pass filters between the DA-converter and the operational amplifier are passive, forth order, RLC filters with a cut-off frequency of 84 MHz. The output stage is a fully differential, current feedback, operational amplifier. At each output there is a 50 Ω output resistor. Usually, only the positive output from the output op-amp is connected to a BNC connector on the front panel of the enclosure. However, it is possible to reconfigure the internal cables to output both parts of the differential pair.

The AUXDAC output from the DA-converter can be used to set av offset of -0.01 V to +1.4 V to the outputs. See hardware.Hardware.set_dac_auxdac().

OUT A, OUT B, OUT C and OUT D

The slow (up to 800 kSamples/sec) output ports are controlled by a single four-channel DA-converter. Each port is then driven by two operational amplifiers, one for generating the proper offset voltage and one for providing gain. The resulting voltage range is -4.1 V to +4.1V. The outputs have 50 Ω output impedance. The output from port OUT A is also connected to an inverting amplifier which makes it possible to achieve a negative copy of the OUT A signal. This can be useful for applications that require a differential signal. Normally the OUT A- and OUT D signals are not connected to a BNC port on the enclosure, but these signals can be made available by reconfiguring the internal cables.

IN1, IN2, IN3 and IN4

Port	Coupling	Input voltage range	Zin differential	Zin Single ended	X	R
1	DC	-0.75 V to +0.75 V	100 Ω	50 Ω	0 Ω	0 Ω
2	DC	-0.75 V to +0.75 V	100 Ω	50 Ω	0 Ω	0 Ω
3	DC	-1 V to + 1 V	100 Ω	50 Ω	0 Ω	0 Ω
2	DC	-1 V to + 1 V	100 Ω	50 Ω	0 Ω	0 Ω

The MLA is typically used in special measurement setups where the signal quality is of crucial importance. Both the noise, linearity and bandwidth must be kept at the absolut optimal values. In order to achieve these goals, the number of active circuits in the signal path has been reduced to a minimum and the AD converters are only driven by one single, fully differential operational amplifier. This amplifier has unity gain and this gain cannot be configured from the software.

Not having a configurable gain of the input stage may be a limitation in some cases. But in situations where the signal quality is important, best results can only be achieved by a using an application specific pre-amplifier anyway. Therefore, having only one more amplifier in the signal path will ensure optimal signal quality. If you don’t have a suitable pre-amplifier for your application, please consult Intermodulation Products.

It is important that the input to the positive and negative arms of the differential inputs is symmetric. If it is not symmetric, the analog gain will be altered which means that the calibration will not be correct. But most notably, the DC-coupled input ports (IN2 and IN4) will have a large offset voltage. (This DC offset originates from the CM pin of the AD-converter which sits at 0.78 V. It should be noted that if you don’t use a preamplifier, but connect the sample directly to one of the ports IN2 or IN4, the sample may be affected by this offset.)

So, for example, if you have a single ended source with 50 Ω output impedance connected to the IN+ port, a 50 Ω resistor should be connected between IN- and ground. This can be done, either by soldering on the PC-board, or connecting a 50 Ω SMA terminator to the IN- port. Most low- to intermediate-frequency preamplifiers have an output impedance close to 0 Ω. In these cases the IN- port should be shorted to ground, again, either by soldering or using an SMA short.