Analog Devices AD605 Evaluation Board AD605-EVALZ AD605-EVALZ Fiche De Données

 

Rev. F | Page 16 of 24 

The basic circuit in Figure 38 shows the connections for one 
channel of the AD605 with a gain range of −14 dB to +34.4 dB. 
The signal is applied at +IN1. The ac coupling capacitors before 
Pin −IN1 and Pin +IN1 should be selected according to the 
required lower cutoff frequency. In this example, the 0.1 μF 
capacitors, together with the 175 Ω of each of the DSX input 
pins, provide a −3 dB high-pass corner of about 9.1 kHz. The 
upper cutoff frequency is determined by the amplifier and is 
40 MHz. 

0

05

41

-03

9

As shown in Figure 38, the output is ac-coupled for optimum 
performance. In the case of connecting to the 10-bit, 40 MSPS 
ADC, 

, ac coupling can be eliminated as long as 

Pin VOCM is biased by the same 3.3 V common-mode voltage 
as the AD9050. 

Pin VREF requires a voltage of 1.25 V to 2.5 V, with gain scaling 
between 40 dB/V and 20 dB/V, respectively. Voltage VGN controls 
the gain; its nominal operating range is from 0.25 V to 2.65 V 
for 20 dB/V gain scaling and 0.125 V to 1.325 V for 40 dB/V 
scaling. When this pin is taken to ground, the channel powers 
down and disables its output. 

Figure 39 shows the two channels of the AD605 connected in 
series to provide a total gain range of 96.8 dB. When R1 and R2 
are shorts, the gain range is from −28 dB to +68.8 dB with a 
slightly reduced bandwidth of about 30 MHz. The reduction in 
bandwidth is due to two identical low-pass circuits being connected 
in series; in the case of two identical single-pole, low-pass filters, 
the bandwidth is reduced by exactly √2. If R1 and R2 are 
replaced by open circuits, that is, Pin FBK1 and Pin FBK2 are left 
unconnected, the gain range shifts up by 28 dB to 0 dB to 96.8 dB. 
As previously noted, the bandwidth of each individual channel is 
reduced by a factor of 5 to about 8 MHz because the gain increased 
by 14 dB. In addition, there is still the √2 reduction because the 
series connection of the two channels results in a final 
bandwidth of the higher gain version of about 6 MHz. 

00

54

1-

0

40

Two other easy combinations are possible to provide a gain 
range of −14 dB to +82.8 dB: make R1 a short and R2 an open, 
or make R1 an open and R2 a short. The bandwidth for both of 
these cases is dominated by the channel that is set to the higher 
gain and is about 8 MHz. From a noise standpoint, the second 
choice is the best because by increasing the gain of the first 
amplifier, the noise of the second amplifier has less of an impact 
on the total output noise. One further observation regarding 
noise is that by increasing the gain, the output noise increases 
proportionally; therefore, there is no increase in signal-to-noise 
ratio. It actually stays fixed. 

It should be noted that by selecting the appropriate values of R1 
and R2, any gain range between −28 dB to +68.8 dB and 0 dB to 
+96.8 dB can be achieved with the circuit in Figure 39. When 
using any value other than shorts and opens for R1 and R2, the 
final value of the gain range depends on the external resistors 
matching the on-chip resistors. Because the internal resistors 
can vary by as much as ±20%, the actual values for a particular 
gain have to be determined empirically. Note that the two channels 
within one part match quite well; therefore, R1 tracks R2 in 
Figure 39. 

C3 is not required because the common-mode voltage at 
Pin OUT1 should be identical to the one at Pin +IN2 and 
Pin −IN2. However, because only 1 mV of offset at the output  
of the first DSX introduces an offset of 53 mV when the second 
DSX is set to the maximum gain of the lowest gain range (34.4 dB), 
and 263 mV when set to the maximum gain of the highest gain 
range (48.4 dB), it is important to include ac coupling to get the 
maximum dynamic range at the output of the cascaded amplifiers. 
C5 is necessary if the output signal needs to be referenced to any 
common-mode level other than half of the supply as is provided 
by Pin OUT2.