Analog Devices AD9740 Evaluation Board AD9740ACP-PCBZ AD9740ACP-PCBZ データシート

 

Rev. B | Page 14 of 32 

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The AD9740 contains a control amplifier that is used to regulate 
the full-scale output current, I

OUTFS

. The control amplifier is 

configured as a V-I converter, as shown in Figure 24, so that its 
current output, I

REF

, is determined by the ratio of the V

REFIO

 and 

an external resistor, R

SET

, as stated in Equation 4. I

REF

 is copied 

to the segmented current sources with the proper scale factor to 
set I

OUTFS

, as stated in Equation 3. 

The control amplifier allows a wide (10:1) adjustment span of 
I

OUTFS

 over a 2 mA to 20 mA range by setting I

REF

 between 

62.5 μA and 625 μA. The wide adjustment span of I

OUTFS

 

provides several benefits. The first relates directly to the power 
dissipation of the AD9740, which is proportional to I

OUTFS

 (see 

the Power Dissipation section). The second relates to a 20 dB 
adjustment, which is useful for system gain control purposes. 

The small signal bandwidth of the reference control amplifier is 
approximately 500 kHz and can be used for low frequency small 
signal multiplying applications. 

The AD9740 provides complementary current outputs, IOUTA 
and IOUTB. IOUTA provides a near full-scale current output, 
I

OUTFS

, when all bits are high (that is, DAC CODE = 1023), while 

IOUTB, the complementary output, provides no current. The 
current output appearing at IOUTA and IOUTB is a function of 
both the input code and I

OUTFS

 and can be expressed as: 

IOUTA = (DAC CODE/1023) × I

 (1) 

IOUTB = (1023 − DAC CODE)/1024 × I

where DAC CODE = 0 to 1023 (that is, decimal representation). 

As mentioned previously, I

OUTFS

 is a function of the reference 

current I

REF

, which is nominally set by a reference voltage, 

V

REFIO

, and external resistor, R

SET

. It can be expressed as: 

 = 32 × I

 (3) 

where 

 = V

/R

 (4) 

 

The two current outputs typically drive a resistive load directly 
or via a transformer. If dc coupling is required, then IOUTA 
and IOUTB should be directly connected to matching resistive 
loads, R

LOAD

, that are tied to analog common, ACOM. Note that 

R

LOAD

 can represent the equivalent load resistance seen by 

IOUTA or IOUTB, as would be the case in a doubly terminated 
50 Ω or 75 Ω cable. The single-ended voltage output appearing 
at the IOUTA and IOUTB nodes is simply 

 = IOUTA × R

 (5) 

 = IOUTB × R

 (6) 

Note that the full-scale value of V

OUTA

 and V

OUTB

 should not 

exceed the specified output compliance range to maintain 
specified distortion and linearity performance. 

 = (IOUTA − IOUTB) × R

 (7) 

Substituting the values of IOUTA, IOUTB, I

REF

, and V

DIFF

 can be 

expressed as: 

 = {(2 × DAC CODE − 1023)/1024}  

(32 × R

/R

) × V

 (8) 

Equation 7 and Equation 8 highlight some of the advantages of 
operating the AD9740 differentially. First, the differential 
operation helps cancel common-mode error sources associated 
with IOUTA and IOUTB, such as noise, distortion, and dc 
offsets. Second, the differential code-dependent current and 
subsequent voltage, V

DIFF

, is twice the value of the single-ended 

voltage output (that is, V

OUTA

 or V

OUTB

), thus providing twice the 

signal power to the load. 

Note that the gain drift temperature performance for a single-
ended (V

OUTA

 and V

OUTB

) or differential output (V

B

DIFF

) of the 

AD9740 can be enhanced by selecting temperature tracking 
resistors for R

LOAD

 and R

SET

 due to their ratiometric relationship, 

as shown in Equation 8. 

The complementary current outputs in each DAC, IOUTA,  
and IOUTB can be configured for single-ended or differential 
operation. IOUTA and IOUTB can be converted into 
complementary single-ended voltage outputs, V

OUTA

 and V

OUTB

, 

via a load resistor, R

LOAD

, as described in the DAC Transfer 

Function section by Equation 5 through Equation 8. The 
differential voltage, V

DIFF

, existing between V

OUTA

 and V

OUTB

, can 

also be converted to a single-ended voltage via a transformer or 
differential amplifier configuration. The ac performance of the 
AD9740 is optimum and specified using a differential 
transformer-coupled output in which the voltage swing at 
IOUTA and IOUTB is limited to ±0.5 V. 

The distortion and noise performance of the AD9740 can be 
enhanced when it is configured for differential operation. The 
common-mode error sources of both IOUTA and IOUTB can 
be significantly reduced by the common-mode rejection of a