Texas Instruments | Interfacing the VCA8617 with High-Speed ADCs | Application notes | Texas Instruments Interfacing the VCA8617 with High-Speed ADCs Application notes

Texas Instruments Interfacing the VCA8617 with High-Speed ADCs Application notes
Application Report
SBAA130 – April 2005
Interfacing the VCA8617 with High-Speed
Analog-to-Digital Converters
Harish Venkataraman, Stephan Baier................................................................... High-Speed Data Group
ABSTRACT
The VCA8617 is an 8-channel variable gain amplifier ideally suited for portable and
mid-range ultrasound applications. Each channel consists of a Low Noise Amplifier
(LNA) and a Variable Gain Amplifier (VGA). The VGA contains two parts: a
voltage-controlled attenuator (VCA) and a programmable gain amplifier (PGA). The
PGA output feeds directly into an integrated 3-pole low-pass Butterworth filter. This
integrated filter prevents the need for an external filter between the VCA and the
analog-to-digital converter (ADC). This application report discusses different methods of
interfacing the VCA8617 with various ADCs, particularly the ADS527x and ADS512x
families.
1
Using the VCA8617 with the ADS527x
The ADS527x family of ADCs from Texas Instruments is a series of octal 3.3V converters that offer
high-performance, 12-bit solutions at various speed nodes, except for the ADS5277, which is a 10-bit,
65MSPS solution.
The VCA8617 output pins sit at a nominal common-mode dc voltage of 1.5V. The input pins of the
ADS527x family sit at 1.5V common-mode dc voltage, as well. This correlation means that it is possible to
dc-couple the VCA8617 with the ADS527x family in a fairly straightforward manner. DC-coupling would
eliminate the space required by 16 capacitors placed between the VCA8617 and any of the ADS527x
devices.
The VCA8617 output impedance is very low. Consequently, a 25Ω to 50Ω resistor is recommended,
placed in series between the VCA8617 and the ADS527x. Best results are obtained when a small
capacitor (5pF to 10pF) is used between the positive and negative inputs of the ADS527x.
The ADS527x family has an internal 600Ω resistor from each input pin to common-mode. The small 25Ω
to 50Ω series resistor, in conjunction with the 600Ω shunt resistor, provides a small attenuation factor.
Users should take this attenuation factor into account when interfacing the VCA8617 with an ADS527x
device. Note also that this 600Ω resistor is fixed.
Figure 1 illustrates this solution.
SBAA130 – April 2005
Interfacing the VCA8617 with High-Speed Analog-to-Digital Converters
1
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Using the VCA8617 with the ADS527x
VCM
Internal Circuit
External Circuit
Channel 1+
Channel 1−
Internal Circuit
600Ω
50Ω
50Ω
AIN1+
10pF
AIN1−
600Ω
VCM
ADS527x
VCA8617
VCM
Channel 8+
Channel 8−
600Ω
50Ω
50Ω
AIN8+
10pF
AIN8−
600Ω
VCM
Figure 1. DC-Coupled Interface Between VCA8617 and ADS527x
While dc-coupling appears to be the simplest possible method to interface the VCA8617 with the
ADS527x, ac-coupling is the recommended solution. There are several reasons why ac-coupling may
provide increased functionality for end-user applications.
First, there is an inherent output common-mode variation associated with the output pins of the VCA8617.
This variation increases as the PGA gain setting increases. Final test measurements show that at the
highest post gain (PG) gain setting of 11, this variation can be as high as 100mV to 150mV. (Note that
most production units fall within ±50mV of 1.5V, which is also within the tolerance of the ADS527x input
pins. However, there are units which may shift slightly more.) This variation decreases as the PG gain
settings decrease; at the lowest PG setting of 00, this variation seems to be less than half the variation
observed at the highest PG gain setting.
One other point of note here is that between the positive and negative output pins, the offset of any one
channel is almost negligible. It is only the absolute value that appears to vary. The problem is that the
offset shift might generate a slight loss in overall dynamic range. This variation is observed over PG gain
settings from channel to channel as well as from device to device. AC-coupling the VCA8617 and the
ADS527x resolves this problem.
Second, there is also a settling time observed at the output of the VCA8617 when the control voltage is
pulsed. The output settles after several time constants; the time constant is set by the internal value of an
RC circuit between the VCA and PGA. AC-coupling at the output can override this internal time constant
and can enable the VCA8617 to recover quickly. The shorter the acceptable time constant, the more
quickly the VCA8617 appears to settle.
An ac-coupling capacitor value of 500pF was tested by Texas Instruments, and appears to provide no
degradation in performance. Keep in mind that there is an internal 600Ω resistor inside the ADS527x, from
each input pin to common-mode.
2
Interfacing the VCA8617 with High-Speed Analog-to-Digital Converters
SBAA130 – April 2005
www.ti.com
Overload Conditions
While the combination of the 500pF capacitor in series with a 50Ω resistor and the internal 600Ω resistor
seems to work well, TI recommends that each user come to his or her own conclusion by trying various
combinations, and choosing that combination which works best to fit the system requirements.
Figure 2 illustrates one approach to this solution.
VCM
Internal Circuit
External Circuit
500pF
50Ω
500pF
50Ω
Channel 1+
Channel 1−
Internal Circuit
600Ω
AIN1+
10pF
AIN1−
600Ω
VCM
ADS527x
VCA8617
VCM
Channel 8+
Channel 8−
500pF
50Ω
500pF
50Ω
600Ω
AIN8+
10pF
AIN8−
600Ω
VCM
Figure 2. AC-Coupled Interface Between VCA8617 and ADS527x
2
Overload Conditions
The VCA8617 output pins can output more than 1VPP in single-ended mode, or greater than 2VPP in
differential mode. Back-to-back clipping diodes across the input and output of the PGA limit the output
signal of the VCA8617 to approximately 0.6V to 0.7V peak on each output. This limited output means that
the clipping diodes turn on when the signal reaches roughly 2.4VPP to 2.5VPP in differential mode. Under
heavy overload conditions, even when the signal is clipped, the VCA8617 can output slightly more than
3VPP. Since the input of the ADS527x can only handle 2VPP in the linear region, the ADS527x ADC will
overload when the VCA8617 output exceeds 2VPP. The ADS527x will recover to 1% accuracy within 3 to 4
clock cycles. This rate means that a recovery to 1% accuracy occurs within 80ns at 50MSPS. If this
recovery time is sufficient, then the above-described solutions should work well in all systems.
If it is necessary to protect the ADS527x ADC further, TI then recommends that the user increase the
value of the series resistor from between 25Ω to 50Ω to approximately 300Ω. This increase ensures that
the ADS527x ADC will never overload, even when the VCA8617 is severely overloaded. The downside to
this solution, however, is that the 300Ω series resistor in conjunction with the 600Ω shunt resistor will
cause a 33% attenuation factor, even at low signal levels when no attenuation is required.
SBAA130 – April 2005
Interfacing the VCA8617 with High-Speed Analog-to-Digital Converters
3
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Interfacing the VCA8617 with the ADS512x family
3
Interfacing the VCA8617 with the ADS512x family
The ADS512x devices are an octal, 1.8V ADC family that provides 10-bit solutions. The input
common-mode of the ADS512x family is 1VPP. Please note that the input pins require external biasing.
Consequently, it is not possible to dc-couple the VCA8617 with the ADS512x family. AC-coupling is the
only option available to users who wish to combine the two devices. TI recommends an ac-coupling
capacitor of at least 300pF and a resistor value of at least 1kΩ to the external common-mode of 1V.
Once again, a small series resistor (20Ω to 50Ω) is recommended between the VCA8617 and the
ADS512x inputs. A 47pF capacitor is also recommended between the positive and negative inputs of the
ADS512x. Figure 3 illustrates the suggested solution.
Internal Circuit
External Circuit
Channel 1+
Internal Circuit
> 300pF 20 − 50Ω
AIN1+
> 1kΩ
47pF
0.1µF
> 300pF
Channel 1−
20 − 50Ω
> 1kΩ
AIN1−
External CM = 1V
ADS512x
VCA8617
Channel 8+
> 300pF
20 − 50Ω
AIN8+
> 1kΩ
47pF
0.1µF
Channel 8−
> 300pF
20 − 50Ω
> 1kΩ
AIN8−
External CM = 1V
Figure 3. AC-Coupled Interface Between VCA8617 and ADS512x
Texas Instruments recommends that users test and validate their own values of capacitors and resistors in
order to identify the combination that works best in their respective applications with the VCA8617 and the
ADS512x.
4
Overload Conditions
The ADS512x family can handle a maximum signal amplitude of 1VPP differential in the linear region. This
means that any signal amplitude in excess of 1VPP at the output of the VCA8617 will overload the
ADS512x.
Since the ADS5121 operates on a low 1.8V supply voltage, its differential input signal range is limited to
1VPP full-scale. Most devices suitable to drive the ADS5121, including the VCA8617, have the capability of
delivering a much larger signal swing on the outputs, and can therefore put the ADS5121 into an overload
condition. Here, two conditions should be considered. The first condition is that the input signal exceeds
the full-scale range, but remains within the supply rails (0V, 1.8V). Second, the input signal exceeds the
supply rails by more than 0.3V, which means that the input signal approaches the absolute maximum
ratings for the device and should be avoided if possible.
4
Interfacing the VCA8617 with High-Speed Analog-to-Digital Converters
SBAA130 – April 2005
www.ti.com
Conclusion
In the first case, if the signal amplitude swings above 1.34V or below 0.82V (approximately), the ADC
output will read either all '1's or all '0's. The output code remains at these endpoints as long as the
overload condition persists, and will not roll over. Once the output signal returns to the normal full-scale
range, the ADS5121 will recover within 20ns (typically) at 40MSPS. In addition, it may take a 6.5 clock
cycles (data latency) to flush the pipeline and discard the invalid data.
Care should be taken to avoid the second condition in which the input signal swings above or below the
supply rails. Such an incident can cause the internal protection diode to conduct and open a
low-impedance path to ac-ground. No dc current will flow because the VCA8617 is ac-coupled; however,
this condition could severely load the VCA outputs.
5
Conclusion
The VCA8617 can be interfaced with TI’s ADS527x family as well as the ADS512x family of high-speed
data converters. While the VCA8617 can be dc-coupled to the ADS527x, it is recommended that
customers ac-couple the VCA8617 with the ADS527x family instead because of the inherent output
common-mode variation in the VCA8617. The VCA8617 must be ac-coupled when interfaced with the
ADS512x.
SBAA130 – April 2005
Interfacing the VCA8617 with High-Speed Analog-to-Digital Converters
5
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