N
Nicholas Kinar
Hello--
I've designed and built a system with a number of AD7690 SAR ADCs.
These particular ADCs have differential inputs. As recommended by the
manufacturer's datasheet and various application notes, I've selected
the ADA4941-1 single-ended to differential driver. This driver has a
given slew rate of 26 Volts/microseconds, and is designed to convert a
single-ended analog input into two phase-shifted outputs.
I've essentially built this particular circuit:
http://www.analog.com/en/verifiedcircuits/CN0032/vc.html
I've modified the circuit for a Vin which swings between 0V and +5V.
This was easy to do based on the design equations given in the
application note. I've also added 0.1uF and 10uF decoupling caps to the
positive and negative voltage rails of the ADA4941-1. An inductor is
also used on these rails to form an LC filter along with the decoupling
capacitors. The inductors are situated close to the voltage inputs.
I've removed the inductors and used a zero ohm resistor to verify that
each inductor is not the source of unwanted oscillations. I have also
tried removing the 10uF tantalum decoupling caps to verify that ESR does
not cause strange behavior.
I was very disappointed to find that there was no SPICE model of the
ADA4941-1 available for download from the Analog Devices website.
However, I have derived some comfort from the fact that the circuit is
"validated."
After building this design, it was more than apparent that my actual
circuit had a very limited slew rate.
(1) The circuit diagram (Figure 1 at the above link) shows two 100nF
(0.1uF) caps attached to nodes Voffset1 and Voffset2. What is the
design purpose of these capacitors? I've found that the presence of
these capacitors caused the output of the ADA4941-1 to be extremely
distorted.
(2) I've used a 2V p-p sine wave as input to the ADA4949-1. At a
frequency of 1kHz, there is very little distortion. However, as I
increase the frequency to 10kHz and to 100kHz, the output wave "shrinks"
and becomes increasingly attenuated around the common-mode voltage of
2.5V. Waveforms with > 2V p-p are attenuated, and the peaks are
sometimes distorted, even when the voltage offset on my function
generator allows for the peaks and troughs to be positive voltages. I
am using +7V and and -4V power rails.
What is the best work-around for this type of circuit behavior? What is
the most probable cause?
Nicholas
I've designed and built a system with a number of AD7690 SAR ADCs.
These particular ADCs have differential inputs. As recommended by the
manufacturer's datasheet and various application notes, I've selected
the ADA4941-1 single-ended to differential driver. This driver has a
given slew rate of 26 Volts/microseconds, and is designed to convert a
single-ended analog input into two phase-shifted outputs.
I've essentially built this particular circuit:
http://www.analog.com/en/verifiedcircuits/CN0032/vc.html
I've modified the circuit for a Vin which swings between 0V and +5V.
This was easy to do based on the design equations given in the
application note. I've also added 0.1uF and 10uF decoupling caps to the
positive and negative voltage rails of the ADA4941-1. An inductor is
also used on these rails to form an LC filter along with the decoupling
capacitors. The inductors are situated close to the voltage inputs.
I've removed the inductors and used a zero ohm resistor to verify that
each inductor is not the source of unwanted oscillations. I have also
tried removing the 10uF tantalum decoupling caps to verify that ESR does
not cause strange behavior.
I was very disappointed to find that there was no SPICE model of the
ADA4941-1 available for download from the Analog Devices website.
However, I have derived some comfort from the fact that the circuit is
"validated."
After building this design, it was more than apparent that my actual
circuit had a very limited slew rate.
(1) The circuit diagram (Figure 1 at the above link) shows two 100nF
(0.1uF) caps attached to nodes Voffset1 and Voffset2. What is the
design purpose of these capacitors? I've found that the presence of
these capacitors caused the output of the ADA4941-1 to be extremely
distorted.
(2) I've used a 2V p-p sine wave as input to the ADA4949-1. At a
frequency of 1kHz, there is very little distortion. However, as I
increase the frequency to 10kHz and to 100kHz, the output wave "shrinks"
and becomes increasingly attenuated around the common-mode voltage of
2.5V. Waveforms with > 2V p-p are attenuated, and the peaks are
sometimes distorted, even when the voltage offset on my function
generator allows for the peaks and troughs to be positive voltages. I
am using +7V and and -4V power rails.
What is the best work-around for this type of circuit behavior? What is
the most probable cause?
Nicholas