Datasheet

PGA870
SBOS436A –DECEMBER 2009–REVISED FEBRUARY 2011 www.ti.com
Using the ADC examples given previously, Table 4 shows sample calculations of the value of R
P
and V
AMP_FS
for
full-scale drive, and then for –1 dBFS.
Table 4. Example R
P
for Various ADCs
ADC
R
IN
|| C
IN
V
AMP_PP
at for 0 V
AMP_PP
V
AMP
V
CM
V
REF
170 MHz R
O
R
P
GAIN GAIN V
ADC_FS
dBFS –1 dBFS
ADC (V
DC
) (V
DC
) (V
DC
) (Ω) (Ω) (Ω) (V/V) (dB) (V
PP
) (V
PP
) (V
PP
)
ADS6149 2.5 1.5 0 216 25 37.5 0.53 –5.57 2 3.80 3.38
ADS58C48/
2.5 0.95 0 195 25 15.3 0.35 –9.21 2 5.78 5.15
ADS4149
ADS58C48/
0 0.95 2.5 195 25 40.8 0.53 –5.43 2 3.74 3.33
ADS4149
(1)
(1) PGA870 operated with ±2.5-V supply.
As Table 4 shows, the signal attenuation as a result of the added resistor network increases as the required
common-mode shift increases. For the ADS6149, the required common-mode level shift is –1 V, from 2.5 V to
1.5 V, and the signal attenuates by 5.57 dB. This difference is a significant signal loss, and the amplifier output
must be increased (either by increasing the PGA870 input or the PGA870 gain) to make up for the loss in order
to drive the full-scale input of the ADC for the highest SNR. At the same time, increasing the amplifier output
swing results in degraded distortion performance as the amplifier output approaches its output range limits.
For the ADS58C48/ADS4149 case with the PGA870 operated with a single 5-V supply, the required level shift
is –1.55 V and the signal attenuates by 9.21 dB. This signal loss cannot be fully recovered by increasing the
PGA870 output: the differential output swing required at the PGA870 output to drive the full-scale range of the
ADS58C48/ADS4149 exceeds the PGA870 output swing capability. Additionally, the distortion performance of
the amplifier is degraded as the output swing increases. In these configurations, the maximum recommended
ADC input is –6 dBFS in order to limit the impact of the additional loading. Another option is to operate the
PGA870 with a split ±2.5-V supply, with the resulting calculations shown in the last row of Table 4. For this
situation, if +2.5 V is used as the V
REF
pull-up voltage, the PGA870 only needs to drive 3.33 V
PP
at its output to
drive the ADS58C48/ADS4149 input to –1 dBFS. See the Operation with Split Supply ±2.5V section for more
details on using the PGA870 with split supplies.
As with any design, testing is recommended to validate whether the result meets the specific design goals.
PGA870 Driving ADS58C48
To illustrate the performance of the PGA870 as an ADC driver, the PGA870 is tested with the ADS58C48 and
bandpass filter designs centered at an operating frequency of 170 MHz. The ADS58C48 is a quad-channel,
11-bit, 200-MSPS ADC with LVDS-compatible digital outputs on six data pairs per channel. The device has
unbuffered analog inputs. There are several key information points to consider when interfacing to the PGA870:
• Unbuffered analog inputs with a frequency-dependent input impedance of Z
IN
= R
IN
|| C
IN
• 0.95-V analog input common-mode voltage
• SNR = 66.1 dBFS (typ) at f
IN
= 17 0MHz
• SFDR = 80 dBc (typ) at f
IN
= 170 MHz
• HD
2
= 82 dBc (typ) at f
IN
= 170 MHz
• HD
3
= 80 dBc (typ) at f
IN
= 170 MHz
• IMD = 83 dBFS (typ) with two-tone input f
IN1
= 185 MHz, f
IN2
= 190 MHz
The ADS58C48EVM is designed for flexible options to ease design work. Used in conjunction with the
TSW1200EVM High-Speed ADC LVDS Evaluation System, it reduces evaluation time to help the designer move
from prototype to production more quickly.
The ADS58C48EVM provides back-to-back input transformers for each of the four analog input channels in order
to convert single-ended test signals to differential when driving the ADCs directly. The Channel D path on the
EVM, however, provides an alternate path (selectable via jumper resistors) for driving the channel with an
onboard PGA870. In this path, a single-ended input test signal can be converted to differential with a single
transformer to drive the PGA870 input. The EVM provides various component pads between the PGA870 and
ADS58C48 input for implementing various filter types. For the latest schematic of the EVM, refer to the
ADS58C48EVM Design Package available through the ADS58C48EVM product page on the TI website.
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Product Folder Link(s): PGA870